Ron Savage > Tree-DAG_Node-Persist-1.06 > Tree::DAG_Node::Persist

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NAME ^

Tree::DAG_Node::Persist - Persist multiple trees in a single db table, preserving child order

Synopsis ^

        my($master) = Tree::DAG_Node::Persist -> new
        (
         context       => 'Master',
         context_col   => 'context',
         dbh           => $dbh,
         id_col        => 'id',
         mother_id_col => 'mother_id',
         name_col      => 'name',
         table_name    => $table_name,
         unique_id_col => 'unique_id',
        );

        my($tree) = build_tree; # Somehow... See the FAQ for help.

        $master -> write($tree);

        my($shrub) = $master -> read;

        # Prune $shrub by adding/deleting its nodes...

        my($offshoot) = Tree::DAG_Node::Persist -> new
        (
         context => 'Offshoot', # Don't use Master or it'll overwrite $tree in the db.
         dbh     => $dbh,
        );

        $offshoot -> write($shrub);

Description ^

Tree::DAG_Node::Persist reads/writes multiple trees from/to a single database table, where those trees are built using Tree::DAG_Node.

See the "FAQ" for details of the table structure.

Distributions ^

This module is available as a Unix-style distro (*.tgz).

See http://savage.net.au/Perl-modules/html/installing-a-module.html for help on unpacking and installing distros.

Installing the module ^

Install Tree::DAG_Node::Persist as you would for any Perl module:

Run:

        cpanm Tree::DAG_Node::Persist

or run:

        sudo cpan Tree::DAG_Node::Persist

or unpack the distro, and then either:

        perl Build.PL
        ./Build
        ./Build test
        sudo ./Build install

or:

        perl Makefile.PL
        make (or dmake)
        make test
        make install

Method: context([$new_value]) ^

Get or set the value to be used in the 'context' column when the tree is written to or read from the database.

Method: context_col([$new_value]) ^

Get or set the value to be used as the name of the 'context' column when the tree is written to or read from the database.

Method: dbh([$new_value]) ^

Get or set the value to be used as the database handle when the tree is written to or read from the database.

Method: id_col([$new_value]) ^

Get or set the value to be used as the name of the 'id' column when the tree is written to or read from the database.

Method: mother_id_col([$new_value]) ^

Get or set the value to be used as the name of the 'mother_id' column when the tree is written to or read from the database.

Method: name_col([$new_value]) ^

Get or set the value to be used as the name of the 'name' column when the tree is written to or read from the database.

Method: new({...}) ^

Returns a new object of type Tree::DAG_Node::Persist.

Key-value pairs in the hashref:

context => $a_string

This is the value to be used in the 'context' column when the tree is written to or read from the database.

This key is optional.

It defaults to '-'.

context_col => $a_string

This is the name to be used for the 'context' column when the tree is written to or read from the database.

This key is optional.

If defaults to 'context'.

dbh => A database handle

This is the database handle to use.

This key-value pair is mandatory.

There is no default.

id_col => $a_string

This is the name to be used for the 'id' column when the tree is written to or read from the database.

This key is optional.

If defaults to 'id'.

mother_id_col => $a_string

This is the name to be used for the 'mother_id' column when the tree is written to or read from the database.

This key is optional.

If defaults to 'mother_id'.

name_col => $a_string

This is the name to be used for the 'name' column when the tree is written to the database.

This key is optional.

If defaults to 'name'.

table_name => $a_string

This is the name of the database table used for reading and writing trees.

This key is optional.

If defaults to 'trees'.

unique_id_col => $a_string

This is the name to be used for the 'unique_id' column when the tree is written to or read from the database.

This key is optional.

If defaults to 'unique_id'.

Method: table name([$new_value]) ^

Get or set the value to be used as the name of the table when the tree is written to or read from the database.

Method: unique_id_col([$new_value]) ^

Get or set the value to be used as the name of the 'unique_id' column when the tree is written to or read from the database.

Method: read([$extra]) ^

Returns a tree of type Tree::DAG_Node read from the database.

If the optional parameter $extra is provided, then it is assumed to be an arrayref of field names.

read($extra) is used in conjunction with write($tree, $extra). See that method for more details.

This code shows how to save and restore an attribute of each node called 'page_id'.

Note: In this code, the [] indicate an arrayref, not optional parameters.

        $object -> write($tree, ['page_id']);

        $shrub = $object -> read(['page_id']);

The test program t/test.t demonstrates usage of this feature.

Method: write_node($node, {...}) ^

This method is called by write(), and - naturally - you'll never call it directly.

Method: write($tree[, $extra]) ^

Writes a tree of type Tree:DAG_Node to the database.

If the optional parameter $extra is provided, then it is assumed to be an arrayref of field names:

o Each field's name is the name of a column in the table
o Each field's value is extracted from the attributes of the node, via the field's name
o The (field name => field value) pairs are written to each record in the table

In particular note that you can store - in a single table - trees which both do and don't have extra fields.

Just ensure the definition of each extra column is flexible enough to handle these alternatives.

The test program t/test.t demonstrates usage of this feature.

This method does not return a meaningful value.

FAQ ^

What is the required table structure?

Firstly, note that the column names used here are the defaults. By supplying suitable parameters to new(), or calling the appropriate method, you can use any column names you wish.

As a minimum, you must have these fields in the table used to hold the trees:

        id $primary_key,
        mother_id integer not null,
        unique_id integer not null,
        context varchar(255) not null,
        name varchar(255) not null

You can generate the $primary_key text using DBIx::Admin::CreateTable, as is done in t/test.t.

What is id?

Strictly speaking, the id field does not have to be a primary key, but it must be unique, because it's used as a hash key when a tree is read in from the database.

The value of id is stored in each node when the tree is read in, whereas the values of context and unique_id are not.

The id of a node can be recovered from the 'attribute' hashref associated with any node, using the code:

        my($id) = ${$node -> attribute}{id} || 0;

Of course, this id (in the 'attribute' hashref) only exists if the tree has been written to the database and read back in. For a brand-new node, which has never been saved, there is no id value by default, hence the '|| 0'. Naturally, you're free to jam some sort of value in there yourself.

What is mother_id?

It is the id of the node which is the mother of the 'current' node. Using 'mother' rather than 'parent', and 'daughter' rather than 'child', is terminology I have adopted from Tree::DAG_Node.

The mother_id of the root of each tree is 0, allowing you to use 'not null' on the definition of mother_id.

This 'not null' convention is adopted from:

        Joe Celko's SQL for Smarties 2nd edition
        Morgan Kaufmann
        1-55860-576-2
        Section 6.9, page 120, Design Advice for NULLs

The mother_id of a node can be recovered from the 'attribute' hashref associated with any node, using the code:

        my($mother) = $node -> mother;
        my($id)     = $mother ? ${$mother -> attribute}{id} : 0;
What is unique_id?

For a given tree (in the database), each node has the same value for context, but a unique value for unique_id.

The reason the id field is not used for this, is that nodes in one tree may be deleted, so that when a second tree is written to the database, if the database reuses ids, the order of ids no longer means anything.

The module writes a node to the database before it writes that node's children. By generating a unique value as the nodes are written, the module guarantees a node's unique_id will be less that the unique_ids of each of its children.

Then, when the nodes are read back in, the database is used to sort the nodes using their unique_id as the sort key.

In this manner, the order of children belonging to a node is preserved.

The field unique_id is only unique for a given tree (in the database). The root of each tree has a unique_id of 1.

The value of id is stored in each node when the tree is read in, whereas the value of context and unique_id are not.

What is context?

You give each tree some sort of identifying string, which is stored in the context field.

For a given tree, all nodes must have the same value for this context field.

Reading a tree means reading all records whose context matches the value you provide.

Writing a tree means:

  • Delete

    All records whose context matches the value you provide are deleted.

  • Insert

    All nodes in the tree are inserted in the table.

The reason for this 2-step process is to avoid depending on ids provided by the database, which may be reused after records are deleted.

By inserting the tree afresh each time, we can ensure the unique_id values for the given tree are generated in such a way that when the records are read back in, sorted by unique_id, each mother node is read before any of its daughters. This makes it easy to insert the incoming data into a new tree in a reliable manner, and to guarantee daughter nodes have their order preseved throughout the write-then-read cycle.

The value of id is stored in each node when the tree is read in, whereas the value of context and unique_id are not.

What is name?

Each node can have any name you wish. See Tree::DAG_Node for details.

The name of a node can be recovered with the name method associated with any node, using the code:

        my($name) = $node -> name;
How do I build a tree from a text file?

See sub build_tree() in t/test.t, and where it's called from.

How do I process a single node?

See sub find_junk() or sub find_node() in t/test.t, and where they're called from.

How do I pretty-print a tree?

See sub pretty_print() in t/test.t, and where it's called from.

How do I run t/test.t?

You can set the environment variables $DBI_DSN, $DBI_USER and $DBI_PASS, and the program will use a table called 'menus'. The default table name is 'trees'.

Or, if $DBI_DSN has no value, the program will use SQLite and a default file (i.e. database) name, in the temp directory.

Machine-Readable Change Log ^

The file CHANGES was converted into Changelog.ini by Module::Metadata::Changes.

See Also ^

Data::NestedSet. This module has its own list of See Also references.

DBIx::Tree::NestedSet. This module has its own list of See Also references.

DBIx::Tree.

DBIx::Tree::Persist.

Tree.

Tree::Persist.

Thanx to the author(s) of Tree::Persist, for various ideas implemented in this module.

Tree::DAG_Node.

Support ^

Email the author, or log a bug on RT:

https://rt.cpan.org/Public/Dist/Display.html?Name=Tree-DAG_Node-Persist.

Author ^

Tree::DAG_Node::Persist was written by Ron Savage <ron@savage.net.au> in 2010.

Home page: http://savage.net.au/index.html.

Copyright ^

Australian copyright (c) 2010, Ron Savage.

        All Programs of mine are 'OSI Certified Open Source Software';
        you can redistribute them and/or modify them under the terms of
        The Artistic License, a copy of which is available at:
        http://www.opensource.org/licenses/index.html
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