Anton Berezin > DBIx-Perlish-0.63 > DBIx::Perlish

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NAME ^

DBIx::Perlish - a perlish interface to SQL databases

VERSION ^

This document describes DBIx::Perlish version 0.63

SYNOPSIS ^

    use DBI;
    use DBIx::Perlish;

    my $dbh = DBI->connect(...);
    DBIx::Perlish::init($dbh);

    # selects:
    my @rows = db_fetch {
        my $x : users;
        defined $x->id;
        $x->name !~ /\@/;
    };

    # sub-queries:
    my @rows = db_fetch {
        my $x : users;
        $x->id <- db_fetch {
            my $t2 : table1;
            $t2->col == 2 || $t2->col == 3;
            return $t2->user_id;
        };
        $x->name !~ /\@/;
    };

    # updates:
    db_update {
        data->num < 100;
        data->mutable;

        data->num = data->num + 1;
        data->name = "xyz";
    };

    # more updates:
    db_update {
        my $d : data;
        $d->num < 100, $d->mutable;

        $d = {
            num  => $d->num + 1,
            name => "xyz"
        };
    };

    # deletes:
    db_delete {
        my $t : table1;
        !defined $t->age  or
        $t->age < 18;
    };

    # inserts:
    my $id = 42;
    db_insert 'users', {
        id   => $id,
        name => "moi",
    };

DESCRIPTION ^

The DBIx::Perlish module provides the ability to work with databases supported by the DBI module using Perl's own syntax for four most common operations: SELECT, UPDATE, DELETE, and INSERT.

By using DBIx::Perlish, you can write most of your database queries using a domain-specific language with Perl syntax. Since a Perl programmer knows Perl by definition, and might not know SQL to the same degree, this approach generally leads to a more comprehensible and maintainable code.

The module is not intended to replace 100% of SQL used in your program. There is a hope, however, that it can be used to replace a substantial portion of it.

The DBIx::Perlish module quite intentionally neither implements nor cares about database administration tasks like schema design and management. The plain DBI interface is quite sufficient for that. Similarly, and for the same reason, it does not take care of establishing database connections or handling transactions. All this is outside the scope of this module.

Ideology

There are three sensible and semi-sensible ways of arranging code that works with SQL databases in Perl:

SQL sprinkling approach

One puts queries wherever one needs to do something with the database, so bits and pieces of SQL are intermixed with the program logic. This approach can easily become an incomprehensible mess that is difficult to read and maintain.

Clean and tidy approach

Everything database-related is put into a separate module, or into a collection of modules. Wherever database access is required, a corresponding sub or method from such a module is called from the main program. Whenever something is needed that the DB module does not already provide, a new sub or method is added into it.

Object-relational mapping

One carefully designs the database schema and an associated collection of classes, then formulates the design in terms of any of the existing object-relational mapper modules like Class::DBI, DBIx::Class or Tangram, then uses objects which perform all necessary queries under the hood. This approach is even cleaner than "clean and tidy" above, but it has other issues. Some schemas do not map well into the OO space. Typically, the resulting performance is an issue as well. The performance issues can in some cases be alleviated by adding hand-crafted SQL in strategic places, so in this regard the object-relational mapping approach can resemble the "clean and tidy" approach.

The DBIx::Perlish module is meant to eliminate the majority of the "SQL sprinkling" style of database interaction. It is also fully compatible with the "clean and tidy" method.

Procedural interface

init()

The init() sub initializes procedural interface to the module.

It accepts named parameters. One parameter, dbh, is required and must be a valid DBI database handler.

All other parameters are silently ignored.

Alternatively, init() can be called with a single positional parameter, in which case it is assumed to be the DBI database handler.

If the supplied database handler is not valid, an exception is thrown.

This procedure does not return anything meaningful.

Examples:

    my $dbh = DBH->connect(...);
    DBIx::Perlish::init(dbh => $dbh);

    my $dbh = DBH->connect(...);
    DBIx::Perlish::init($dbh);

Special treatment of the $dbh variable

If the user did not call init() before issuing any of the db_fetch {}, db_update {}, db_delete {} or db_insert {}, those functions look for one special case before bailing out.

Namely, they try to locate a variable my $dbh, our $dbh, or caller's package global $dbh, in that order, in the scope in which they are used. If such variable is found, and if it contains a valid DBI database handler, they will use it for performing the actual query. This allows one to write something like that, and expect the module to do the right thing:

    my $dbh = DBI->connect(...);
    my @r = db_fetch { users->name !~ /\@/ };

Initially, the author did not recommend relying on this feature in the production code on a theory that it is `magical' and makes assumptions about names used outside of the module itself. The author recommended to always use the init() subroutine.

However, the experience of using the DBIx::Perlish module in production environment has shown that the assumption about the name of the database handle is true in almost all cases in practice (but see "Working with multiple database handles" below), and that the magical nature of the feature does not hinder, indeed helps writing code in a style that is easy to maintain. In fact, there is often less of an action-at-a-distance effect when relying on the implicit $dbh instead of using the init() sub.

db_fetch {}

The db_fetch {} function queries and returns data from the database.

The function parses the supplied query sub, converts it into the corresponding SQL SELECT statement, and executes it.

What it returns depends on two things: the context and the return statement in the query sub, if any.

If there is a return statement which specifies exactly one column, and db_fetch {} is called in the scalar context, a single scalar representing the requested column is returned for the first row of selected data. Example:

    my $somename = db_fetch { return user->name };

Borrowing DBI's terminology, this is analogous to

    my $somename =
        $dbh->selectrow_array("select name from user");

If there is a return statement which specifies exactly one column, and db_fetch {} is called in the list context, an array containing the specified column for all selected rows is returned. Example:

    my @allnames = db_fetch { return user->name };

This is analogous to

    my @allnames =
        @{$dbh->selectcol_arrayref("select name from user")};

When there is no return statement, or if the return statement specifies multiple columns, then an individual row is represented by a hash reference with column names as the keys.

In the scalar context, a single hashref is returned, which corresponds to the first row of selected data. Example:

    my $h = db_fetch { my $u : user };
    print "name: $h->{name}, id: $h->{id}\n";

In DBI parlance that would look like

    my $h = $dbh->selectrow_hashref("select * from user");
    print "name: $h->{name}, id: $h->{id}\n";

In the list context, an array of hashrefs is returned, one element for one row of selected data:

    my @users = db_fetch { my $u : user };
    print "name: $_->{name}, id: $_->{id}\n" for @users;

Again, borrowing from DBI, this is analogous to

    my @users = @{$dbh->selectall_arrayref("select * from user",
        {Slice=>{}})};
    print "name: $_->{name}, id: $_->{id}\n" for @users;

There is also a way to specify that one or several of the return values are the key fields, to obtain a behavior similar to that of the DBI's selectall_hashref() function. A return value is a key field if it is prepended with -k:

    my %data = db_fetch {
        my $u : users;
        return -k $u->name, $u;
    };

This is somewhat analogous to

    my %data = %{$dbh->selectall_hashref(
      "select name, * from users", "name")};

If the db_fetch {} containing key fields is called in the scalar context, it returns a hash reference instead of a hash. In both cases the complete result set is returned.

This is different from calling the db_fetch {} without key fields in the scalar context, which always returns a single row (or a single value), as explained above.

The individual results in such a result set will be hash references if the return statement specifies more than one column (not counting the key fields), or a simple value if the return statement specifies exactly one column in addition to the key fields. For example,

   my %data = db_fetch {
      my $u : user;
      return -k $u->id, $u;
   };
   print "The name of the user with ID 42 is $data{42}{name}\n";

but:

   my %data = db_fetch {
      my $u : user;
      return -k $u->id, $u->name;
   };
   print "The name of the user with ID 42 is $data{42}\n";

In any case, the key fields themselves are never present in the result, unless they were specified in the return statement independently.

The db_fetch {} function will throw an exception if it is unable to find a valid database handle to use, or if it is unable to convert its query sub to SQL.

In addition, if the database handle is configured to throw exceptions, the function might throw any of the exceptions thrown by DBI.

"Subqueries" are permitted in db_fetch's query subs.

Please see "Query sub syntax" below for details of the syntax allowed in query subs.

The db_fetch {} function is exported by default.

db_select {}

The db_select {} function is an alias to the db_fetch {}. It is exported by default.

db_update {}

The db_update {} function updates rows of a database table.

The function parses the supplied query sub, converts it into the corresponding SQL UPDATE statement, and executes it.

The function returns whatever DBI's do method returns.

The function will throw an exception if it is unable to find a valid database handle to use, or if it is unable to convert its query sub to SQL.

In addition, if the database handle is configured to throw exceptions, the function might throw any of the exceptions thrown by DBI.

A query sub of the db_update {} function must refer to precisely one table (not counting tables referred to by subqueries).

Neither return statements nor last statements are allowed in the db_update {} function's query subs.

An attempt to call the db_update {} function with no filtering expressions in the query sub will throw an exception since such is very likely a dangerous mistake. To allow such an update to proceed, include an exec call with no parameters anywhere in the query sub.

"Subqueries" are permitted in db_update's query subs.

Please see "Query sub syntax" below for details of the syntax allowed in query subs.

Examples:

    db_update {
        tbl->id == 41;
        tbl->id = tbl->id - 1;
        tbl->name = "luff";
    };

    db_update {
        tbl->id = 42;
                exec;  # without this an exception is thrown
    };

    db_update {
        my $t : tbl;
        $t->id == 40;
        $t = {
            id   => $t->id + 2,
            name => "LIFF",
        };
    };

    db_update {
        tbl->id == 40;
        tbl() = {
            id   => tbl->id + 2,
            name => "LIFF",
        };
    };

The db_update {} function is exported by default.

db_delete {}

The db_delete {} function deletes data from the database.

The db_delete {} function parses the supplied query sub, converts it into the corresponding SQL DELETE statement, and executes it.

The function returns whatever DBI's do method returns.

The function will throw an exception if it is unable to find a valid database handle to use, or if it is unable to convert its query sub to SQL.

In addition, if the database handle is configured to throw exceptions, the function might throw any of the exceptions thrown by DBI.

A query sub of the db_delete {} function must refer to precisely one table (not counting tables referred to by subqueries).

Neither return statements nor last statements are allowed in the db_delete {} function's query subs.

An attempt to call the db_delete {} function with no filtering expressions in the query sub will throw an exception since such is very likely a dangerous mistake. To allow such a delete to proceed, include an exec call with no parameters anywhere in the query sub.

"Subqueries" are permitted in db_delete's query subs.

Please see "Query sub syntax" below for details of the syntax allowed in query subs.

Examples:

    db_delete { $x : users; exec; } # delete all users

    # delete with a subquery
    db_delete {
        my $u : users;
        $u->name <- db_fetch {
            visitors->origin eq "Uranus";
            return visitors->name;
        }
    }

The db_delete {} function is exported by default.

db_insert()

The db_insert() function inserts rows into a database table.

This function is different from the rest because it does not take a query sub as the parameter.

Instead, it takes a table name as its first parameter, and any number of hash references afterwards.

For each specified hashref, a new row is inserted into the specified table. The resulting insert statement specifies hashref keys as the column names, with corresponding values taken from hashref values. Example:

    db_insert 'users', { id => 1, name => "the.user" };

A value can be a call to the exported sql() function, in which case it is inserted verbatim into the generated SQL, for example:

    db_insert 'users', {
        id => sql("some_seq.nextval"),
        name => "the.user"
    };

The function returns the number of insert operations performed. If any of the DBI insert operations fail, the function returns undef, and does not perform remaining inserts.

The function will throw an exception if it is unable to find a valid database handle to use.

In addition, if the database handle is configured to throw exceptions, the function might throw any of the exceptions thrown by DBI.

The db_insert {} function is exported by default.

union()

This is a helper sub which is meant to be used inside query subs. Please see "Compound queries' statements" for details. The union() can be exported via :all import declaration.

intersect()

This is a helper sub which is meant to be used inside query subs. Please see "Compound queries' statements" for details. The intersect() can be exported via :all import declaration.

except()

This is a helper sub which is meant to be used inside query subs. Please see "Compound queries' statements" for details. The except() can be exported via :all import declaration.

quirk()

Unfortunately it is not always possible to generate an SQL statement which is valid for different DBI drivers, even when the DBIx::Perlish module has the knowledge about what driver is in use.

The quirk() sub exists to alleviate this problem in certain situations by registering "quirks". Please avoid using it if possible.

It accepts at least two positional parameters. The first parameter is the DBI driver flavor. The second parameter identifies a particular quirk. The rest of parameters are quirk-dependent.

It is a fatal error to attempt to register a quirk that is not recognized by the module.

Currently only Oracle has any quirks, which are listed below:

table_func_cast

When table functions are used in Oracle, one sometimes gets an error "ORA-22905: cannot access rows from a non-nested table item". The solution recommended by Oracle is to do an explicit type cast to a correct type. Since the DBIx::Perlish module has no way of knowing what the correct type is, it needs a little help. The table_func_cast quirk requires two extra parameters, the name of a table function and the type to cast it to.

$SQL and @BIND_VALUES

The DBIx::Perlish module provides two global variables (not exported) to aid in debugging. The $DBIx::Perlish::SQL variable contains the text of the SQL which was generated during the most recent invocation of one of db_fetch {}, db_update {}, or db_delete {}. The @DBIx::Perlish::BIND_VALUES array contains the bind values to be used with the corresponding SQL code.

Query sub syntax

The important thing to remember is that although the query subs have Perl syntax, they do not represent Perl, but a specialized "domain specific" database query language with Perl syntax.

A query sub can consist of the following types of statements:

The order of the statements is generally not important, except that table variables have to be declared before use.

Table variables declarations

Table variables declarations allow one to associate lexical variables with database tables. They look like this:

    my $var : tablename;

It is possible to associate several variables with the same table; this is the preferable mechanism if self-joins are desired.

In case the table name is not known until runtime, it is also possible to write for example

    my $var : table = $data->{tablename};

In this case the attribute "table" must be specified verbatim, and the name of the table is taken from the right-hand side of the assignment.

Database schemas ("schemaname.tablename") are supported in several different ways:

Using the runtime mechanism described above:
    my $tabnam = "schemaname.tablename";
    db_fetch {
        my $t : table = $tabnam;
    };
Using a similar verbatim "table" attribute with a string constant:
    my $t : table = "schemaname.tablename";
Using attribute argument with the verbatim "table" attribute:
    my $t : table(schemaname.tablename);
Using schema name as the attribute and table name as its argument:
    my $t : schemaname(tablename);

Last, but not least, a combination of verbatim "table" attribute with a nested "db_fetch {}" can be used to implement inline views:

    my $var : table = db_fetch { ... };

In this case a select statement corresponding to the nested "db_fetch {}" will represent the table. Please note that not all database drivers support this, although at present the DBIx::Perlish module does not care and will generate SQL which will subsequently fail to execute.

Another possibility for declaring table variables is described in "Statements with label syntax".

Please note that "db_update {}" and "db_delete {}" must only refer to a single table.

Query filter statements

Query filter statements have a general form of Perl expressions. Binary comparison operators, logical "or" (both high and lower precedence form), matching operators =~ and !~, binary arithmetic operators, string concatenation, defined(expr), and unary ! are all valid in the filters. There is also a special back-arrow, "comes from" <- binary operator used for matching a column to a set of values, and for subqueries.

Individual terms can refer to a table column using dereferencing syntax (one of tablename->column, $tablevar->column, tablename->$varcolumn, or $tablevar->$varcolumn), to an integer, floating point, or string constant, to a function call, to next statement with an argument, or to a scalar value in the outer scope (simple scalars, hash elements, or dereferenced hashref elements chained to an arbitrary depth are supported).

Inside constant strings, table column specifiers are interpolated; the result of such interpolation is represented as a sequence of explicit SQL concatenation operations. The variable interpolation syntax is somewhat different from normal Perl rules, which does not interpolate method calls. So it is perfectly legal to write

    return "abc $t->name xyz";

When it is impossible to distinguish between the column name and the following characters, the hash element syntax must be used instead:

    return "abc$t->{name}xyz";

Of course, one may want to avoid the trouble altogether and use explicit Perl concatenation in such cases:

    return "abc" . $t->name . "xyz";

Please note that specifying column names as hash elements is only valid inside interpolated strings; this may change in the future versions of the module.

Please also note that column specifiers of tablename->column form cannot be embedded into strings; again, use explicit Perl concatenation in such cases.

Function calls can take an arbitrary number of arguments. Each argument to a function must currently be a term, although it is expected that more general expressions will be supported in the future. The function call appear verbatim in the resulting SQL, with the arguments translated from Perl syntax to SQL syntax. For example:

    lower($t1->name) eq lower($t2->lastname);

Some of the functions are handled specially:

lc and uc

The Perl builtins lc and uc are translated into lower and upper, respectively.

extract

A two-argument form of the extract function, where the first argument is a constant string, will be converted into the form understood by the SQL standard. For example,

    extract(day => $t->field)

will be converted into something like

    EXTRACT(DAY FROM t01.field)

as is required.

Another special case is when sql() function (with a single parameter) is called. In this case the parameter of the function call inserted verbatim into the generated SQL, for example:

    db_update {
        tab->state eq "new";
        tab->id = sql "some_seq.nextval";
    };

There is also a shortcut when one can use backquotes for verbatim SQL pieces:

    db_update {
        tab->state eq "new";
        tab->id = `some_seq.nextval`;
    };

A next statement with a (label) argument is interpreted as an operator of getting the next value out of a sequence, where the label name is the name of the sequence. Syntax specific to the DBI driver will be used to represent this operation. It is a fatal error to use such a statement with DBI drivers which do not support sequences. For example, the following is exactly equivalent to the example above, except it is more portable:

    db_update {
        tab->state eq "new";
        tab->id = next some_seq;
    };

The "comes from" <- binary operator can be used in the following manner:

    my @ary = (1,2,3);
    db_fetch {
        tab->id  <-  @ary;
    };

This is equivalent to SQL's IN list operator, where the list comes from the @ary array. An array reference or an anonymous array can also be used in place of the @ary here.

The <- operator can also be used with "Subqueries", below.

Return statements

Return statements determine which columns are returned by a query under what names. Each element in the return statement can be either a reference to the whole table, an expression involving table columns, or a string constant, in which case it is taken as an alias to the next element in the return statement:

    return ($table->col1, anothername => $table->col2);

If an element is a reference to the whole table, it is understood that all columns from this table are returned:

    return ($t1->col1, $t1->col2, $t2);

Table references cannot be aliased by a name.

One can also specify a "distinct" or "DISTINCT" string constant in the beginning of the return list, in which case duplicated rows will be eliminated from the result set.

It is also permissible to use a next operator with a label argument (see above) in return statements:

    return next some_seq;

Return statements are only valid in "db_fetch {}".

Query subs representing subqueries using the reverse arrow notation must have exactly one return statement returning exactly one column (see "Subqueries" below).

Assignments

Assignments can take two form: individual column assignments or bulk assignments. The former must have a reference to a table column on the left-hand side, and an expression like those accepted in filter statements on the right-hand side:

    table1->id = 42;
    $t->column = $t->column + 1;

The bulk assignments must have a table specifier on the left-hand side, and a hash reference on the right-hand side. The keys of the hash represent column names, and the values are expressions like those in the individual column assignments:

    $t = {
        id     => 42,
        column => $t->column + 1
    };

or

    tablename() = {
        id     => 42,
        column => tablename->column + 1
    };

Please note a certain ugliness in tablename() in the last example, so it is probably better to either use table vars, or stick to the single assignment syntax of the first example.

It is possible to intermix hashes and hashrefs dereferencings with verbatim key/value pairs in bulk assignments:

    $t = {
        id     => 42,
        column => $t->column + 1,
        %$hashref_from_outer_scope
    };

Please note that the right hand side of the bulk assignment must be an anonymouse hash reference. Thus, the following is invalid:

    $t = $hashref_from_outer_scope;

Instead, write

    $t = {%$hashref_from_outer_scope};

The latter emphasizes the fact that this is the bulk assignment, which is not clear from the former statement.

Assignment statements are only valid in "db_update {}".

Result limiting and ordering statements

The last command can be used to limit the number of results returned by a fetch operation.

If it stands on its own anywhere in the query sub, it means "stop after finding the first row that matches other filters", so it is analogous to LIMIT 1 in many SQL dialects.

It can also be used in conjunction with a range .. operator, so that

    last unless 5..20;

is equivalent to

    OFFSET 5 LIMIT 16

The sort builtin can be used to specify the desired order of the results:

    sort $t->col1, $t->col2;

is equivalent to

    ORDER BY col1, col2

In order to support the ordering direction, the sort expressions can be preceded by a literal string which must satisfy the pattern /^(asc)/i (for ascending order, which is the default), or /^(desc)/i for descending order:

    sort desc => $t->col1, asc => $t->col2;

is equivalent to

    ORDER BY col1 DESC, col2

Result limiting and ordering statements are only valid in "db_fetch {}".

Conditional statements

There is a limited support for parse-time conditional expressions.

At the query sub parsing stage, if the conditional does not mention any tables or columns, and refers exclusively to the values from the outer scope, it is evaluated, and the corresponding filter (or any other kind of statement) is only put into the generated SQL if the condition is true.

For example,

    my $type = "ICBM";
    db_fetch {
        my $p : products;
        $p->type eq $type if $type;
    };

will generate the equivalent to select * from products where type = 'ICBM', while the same code would generate just select * from products if $type were false.

The same code could be written with a real if statement as well:

    my $type = "ICBM";
    db_fetch {
        my $p : products;
        if ($type) {
            $p->type eq $type;
        }
    };

Similarly,

    my $want_z = 1;
    db_fetch {
        my $p : products;
        return $p->x, $p->y         unless $want_z;
        return $p->x, $p->y, $p->z  if     $want_z;
    };

will generate the equivalent of select x, y from products when $want_z is false, and select x, y, z from products when $want_z is true.

Statements with label syntax

There is a number of special labels which query sub syntax allows.

Specifying label distinct: anywhere in the query sub leads to duplicated rows being eliminated from the result set.

Specifying label limit: followed by a number (or a scalar variable representing a number) limits the number of rows returned by the query.

Specifying label offset: followed by a number N (or a scalar variable representing a number N) skips first N rows from the returned result set.

Specifying label order:, orderby:, order_by:, sort:, sortby:, or sort_by:, followed by a list of expressions will sort the result set according to the expressions. For details about the sorting criteria see the documentation for ORDER BY clause in your SQL dialect reference manual. Before a sorting expression in a list one may specify one of the string constants "asc", "ascending", "desc", "descending" to alter the sorting order, for example:

    db_fetch {
        my $t : tbl;
        order_by: asc => $t->name, desc => $t->age;
    };

Specifying label group:, groupby:, or group_by:, followed by a list of column specifiers is equivalent to the SQL clause GROUP BY col1, col2, ....

The module implements an experimental feature which in some cases allows one to omit the explicit group_by: label. If there is an explicit return statement which mentions an aggregate function alongside "normal" column specifiers, and that return statement does not reference the whole table, and the explicit group_by: label is not present in the query, the DBIx::Perlish module will generate one automatically. For example, the following query:

    db_fetch {
        my $t : tab;
        return $t->name, $t->type, count($t->age);
    };

will execute the equivalent of the following SQL statement:

  select name, type, count(age) from tab group by name, type

The avg(), count(), max(), min(), and sum() functions are considered to be aggregate.

Similarly, using an aggregate function in a filtering expression will lead to automatic introduction of a HAVING clause:

    db_fetch {
        my $w : weather;
        max($w->temp_lo) < 40;
        return $w->city;
    };

will translate into an equivalent of

    select city from weather group by city having max(temp_lo) < 40

Specifying label table: followed by a lexical variable declaration, followed by an assignment introduces an alternative table declaration syntax. The value of the expression on the right hand side of the assignment is taken to be the name of the table:

    my $data = { table => "mytable" };
    db_fetch {
        table: my $t = $data->{table};
    };

This is useful if you don't know the names of your table until runtime.

All special labels are case insensitive.

Special labels are only valid in "db_fetch {}".

Compound queries' statements

The SQL compound queries UNION, INTERSECT, and EXCEPT are supported using the following syntax:

    db_fetch {
        {
            ... normal query statements ...
        }
        compound-query-keyword
        {
            ... normal query statements ...
        }
    };

Here compound-query-keyword is one of union, intersect, or except.

This feature will only work if the use statement for the DBIx::Perlish module was written with :all export declaration, since union, intersect, and except are subs that are not exported by default by the module.

It is the responsibility of the programmer to make sure that results of the individual queries used in a compound query are compatible with each other.

Subqueries

It is possible to use subqueries in "db_fetch {}", "db_update {}", and "db_delete {}".

There are two variants of subqueries. The first one is a call, as a complete statement, to "db_fetch {}" anywhere in the body of the query sub. This variant corresponds to the EXISTS (SELECT ...) SQL construct, for example:

    db_delete {
        my $t : table1;
        db_fetch {
            $t->id == table2->table1_id;
        };
    };

Another variant corresponds to the column IN (SELECT ...) SQL construct. It uses a special syntax with back-arrow <- (read it as "comes from"), which signifies that the column specifier on the left gets its values from whatever is returned by a "db_fetch {}" on the right:

    db_delete {
        my $t : table1;
        $t->id  <-  db_fetch {
            return table2->table1_id;
        };
    };

This variant puts a limitation on the return statement in the sub-query query sub. Namely, it must contain a return statement with exactly one return value.

If the right-hand side of the "comes from" operator is a function call, the function is assumed to be a function potentially returning a set of values, or a "table function", in Oracle terminology. Such construct is converted into a driver-dependent subselect involving the table function:

    db_fetch {
        tbl->id  <-  tablefunc($id);
    };

Joins

Joins are implemented similar to subqueries, using embedded db_fetch call to specify a join condition. The join syntax is one of (the last two are equivalent):

    join $t1 BINARY_OP $t2;
    join $t1 BINARY_OP $t2 => db_fetch { CONDITION };
    join $t1 BINARY_OP $t2 <= db_fetch { CONDITION };

where CONDITION is an arbitrary expression using fields from $t1 and $t2 , and BINARY_OP is one of *,+,x,&,|,<,> operators, which correspond to the following standard join types:

Inner join

This corresponds to either of *, &, and x operators. The db_fetch {} condition for inner join may be omitted, in which case it degenerates into a cross join.

Full outer join

It is specified with + or |. The DBIx::Perlish module does not care that some database engines do not support full outer join, nor does it try to work around this limitation.

Left outer join

<

Right outer join

>

Example:

    my $x : x;
    my $y : y;
    join $y * $x => db_fetch { $y-> id == $x-> id };

Object-oriented interface

new()

Constructs and returns a new DBIx::Perlish object.

Takes named parameter.

One parameter, dbh, is required and must be a valid DBI database handler.

Another parameter which the new() understands is quirks, which, if present, must be a reference to an array of anonymous arrays, each corresponding to a single call to quirk(). Please see quirk() for details.

Can throw an exception if the supplied parameters are incorrect.

fetch()

An object-oriented version of "db_fetch {}".

update()

An object-oriented version of "db_update {}".

delete()

An object-oriented version of "db_delete {}".

insert()

An object-oriented version of "db_insert()".

sql()

Takes no parameters. Returns the SQL string, most recently generated by database queries performed by the object. Returns undef if there were no queries made thus far.

Example:

    $db->query(sub { $u : users });
    print $db->sql, "\n";

The sql() sub can also be called in a procedural fashion, in which case it serves the purpose of injecting verbatim pieces of SQL into query subs (see "Query filter statements") or into the values to be inserted via "db_insert()".

The sql() function is exported by default.

bind_values()

Takes no parameters. Returns an array of bind values that were used in the most recent database query performed by the object. Returns an empty array if there were not queries made thus far.

Example:

    $db->query(sub { users->name eq "john" });
    print join(", ", $db->bind_values), "\n";

quirk()

An object-oriented version of "quirk()".

Working with multiple database handles

There are several ways in which the DBIx::Perlish module can be used with several different database handles within the same program:

Using object-oriented interface

The advantage of this approach is that there is no confusion about which database handle is in use, since a DBIx::Perlish object is always created with an explicit database handle as a parameter to "new()".

The obvious disadvantage is that one has to explicitly use "sub" when specifying a query sub, so the syntax is unwieldy.

Switching handles with "init()"

This will work, but it will add quite a bit of clutter to the code, especially if queries to multiple databases are intermixed with each other.

Switching handles via manipulation of the $dbh variable

This has the same disadvantage as the previous method. Besides, it looks like vodoo.

Using special import syntax

It is possible to import differently named specialized versions of the subs normally exported by the DBIx::Perlish module, which will use specified database handle. The syntax is as follows:

    use DBIx::Perlish;
    my $dbh = DBI->connect(...);

    my $foo_dbh = DBI->connect(...);
    use DBIx::Perlish prefix => "foo", dbh => \$foo_dbh;

    my $bar_dbh = DBI->connect(...);
    use DBIx::Perlish prefix => "bar", dbh => \$bar_dbh;

    my @default =  db_fetch { ... };
    my @foo     = foo_fetch { ... };
    my @bar     = bar_fetch { ... };

The syntax and semantics of such specialized versions is exactly the same as with the normal "db_fetch {}", "db_select {}", "db_update {}", "db_delete {}", and "db_insert()", except that they use the database handle specified in the use statement for all operations. As can be seen from the example above, the normal versions still work as intended, employing the usual mechanisms for determining which handle to use.

Database driver specifics

The generated SQL output can differ depending on the particular database driver in use.

MySQL

Native MySQL regular expressions are used if possible and if a simple LIKE won't suffice.

Oracle

The function call sysdate() is transformed into sysdate (without parentheses).

Selects without table specification are assumed to be selects from DUAL, for example:

    my $newval = db_fetch { return `tab_id_seq.nextval` };

Table functions in Oracle are handled specially.

There are quirks (see "quirk()") that can be registered for Oracle driver.

Postgresql

Native Postgresql regular expressions are used if possible and if a simple LIKE won't suffice.

The same applies to PgLite, which is a Postgresql-like wrapper around SQLite. In this case, "native" PgLite regular expressions are actually native Perl regular expressions, but the DBIx::Perlish module pretends it does not know about it.

SQLite

Native Perl regular expressions are used with SQLite even for simple match cases, since SQLite does not know how to optimize LIKE applied to an indexed column with a constant prefix.

Implementation details and more ideology

To achieve its purpose, this module uses neither operator overloading nor source filters.

The operator overloading would only work if individual tables were represented by Perl objects. This means that an object-relational mapper like Tangram can do it, but DBIx::Perlish cannot.

The source filters are limited in other ways: the modules using them are often incompatible with other modules that also use source filtering, and it is very difficult to do source filtering when any degree of flexibility is required. Only perl can parse Perl!

The DBIx::Perlish module, on the other hand, leverages perl's ability to parse Perl and operates directly on the already compiled Perl code. In other words, it parses the Perl op tree (syntax tree).

The idea of this module came from Erlang. Erlang has a so called list comprehension syntax, which allows one to generate lists using generator expressions and to select the list elements using filter expressions. Furthermore, the authors of the Erlang database, Mnesia, hijacked this syntax for the purpose of doing database queries via a mechanism called parse transform. The end result was that the database queries in Erlang are expressed by using Erlang's own syntax.

I found this approach elegant, and thought "why something like this cannot be done in Perl"?

CONFIGURATION AND ENVIRONMENT ^

DBIx::Perlish requires no configuration files or environment variables.

Running under Devel::Cover

When the DBIx::Perlish module detects that the current program is being run under Devel::Cover, it tries to cheat a little bit and feeds Devel::Cover with false information to make those query subs which were parsed by the module to appear "covered".

This is done because the query subs are never executed, and thus would normally be presented as "not covered" by the Devel::Cover reporter. Although a developer has no trouble deciding to ignore such "red islands", he has to perform this decision every time he looks at the coverage data, which tends to become annoying rather quickly.

Currently, only statement and sub execution data are faked.

DEPENDENCIES ^

The DBIx::Perlish module needs at least perl 5.8.2, quite possibly a somewhat higher version. I have only tested it with 5.8.4, 5.8.8, 5.8.9, and 5.10.0, while the CPAN testers, http://www.cpantesters.org/distro/D/DBIx-Perlish.html , provided much better coverage.

This module requires DBI to do anything useful.

In order to support the special handling of the $dbh variable, PadWalker needs to be installed.

Other modules used used by DBIx::Perlish are included into the standard Perl distribution.

INCOMPATIBILITIES ^

Starting with version 0.54 the handling of key fields (return -k $t->field) has incompatibly changed. The previous behavior was to always return individual results as hash references, even when only one column (not counting the key fields) was specified in the return statement. The current behavior is to return simple values in this case.

If you use DBIx::Perlish together with HTML::Mason, you are likely to see warnings "Useless use of ... in void context" that Mason helpfully converts into fatal errors.

To fix this, edit your handler.pl and add the following line:

  $ah->interp->ignore_warnings_expr("(?i-xsm:Subroutine .* redefined|Useless use of .+ in void context)");

Here $ah must refer to an instance of HTML::Mason::ApacheHandler class.

Mason is to blame for this, since it disregards warnings' handlers installed by other modules.

BUGS AND LIMITATIONS ^

No bugs have been reported.

Please report any bugs or feature requests to bug-dbix-perlish@rt.cpan.org, or through the web interface at http://rt.cpan.org.

A number of features found in many SQL dialects is not supported.

The module cannot handle more than 100 tables in a single query sub.

Although variables closed over the query sub can be used in it, only simple scalars, hash elements, and dereferenced hasref elements are understood at the moment.

If you would like to see something implemented, or find a nice Perlish syntax for some SQL feature, please let me know!

AUTHOR ^

Anton Berezin <tobez@tobez.org>

ACKNOWLEDGEMENTS ^

Special thanks to Dmitry Karasik, who contributed code and syntax ideas on several occasions, and with whom I spent considerable time discussing this module.

I would also like to thank Henrik Andersen, Mathieu Arnold, Phil Regnauld, and Lars Thegler, for discussions, suggestions, bug reports and code contributions.

This work is in part sponsored by Telia Denmark.

SUPPORT ^

There is also the project website at http://dbix-perlish.tobez.org/

LICENSE AND COPYRIGHT ^

Copyright (c) 2007-2013, Anton Berezin <tobez@tobez.org>. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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