File: README.rdoc [dm-is-tree-1.2.0 Documentation]

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# dm-is-tree

DataMapper plugin enabling easy creation of tree structures from your DM models.

This requires a foreign key property for your model, which by default would be called `:parent_id`.

## Installation

### Stable

Install the `dm-is-tree` gem.

    $ (sudo)? gem install dm-is-tree

### Edge

Download or clone `dm-is-versioned` from Github.

    $ cd /path/to/dm-is-tree

    $ rake install            # will install dm-is-tree

## Getting started

To start using this gem, just...

    require 'dm-is-tree'

Lets say we have a Category model...

    class Category
      include DataMapper::Resource
      property :id,         Serial
      property :name,       String
    end

...and we want to have a tree structure within it, something like this:

    the_parent
      +- child
          +- grandchild1
          +- grandchild2

To achieve this we just add the following to the model:

    class Category
      <snip...>

      is :tree, :order => :name

    end

    # No need to define the :parent_id property, it will be added automatically
    property :parent_id,  Integer

This will automatically add the following to your model:

### Instance Methods

- `#parent` / `#parent=`

- `#children` / `#children=`

- `#siblings`

- `#generation`

- `#ancestors`

  - also aliased as `#self_and_siblings` for those used to AR's :acts_as_tree

- `#root`

  - also aliased as `#first_root` for those used to AR's :acts_as_tree

### Class Methods

- `#first_root`

- `#roots`

## Configuration Options

Before we go onto the usage examples, a few quick words about configuration options available:

### `:child_key`

Specifies the column name to use for tracking of the tree (default: `#parent_id`).

    class Category
      <snip...>

      is :tree, :child_key => :some_other_foreign_key_id

    end

### `:model`

Specifies the name of the Model to use for the tree (default: Model class name defined in)

    class Category
      <snip...>

      is :tree, :model => 'SomeStrangeModelName'

    end

### `:order` [Optional]

Specifies the sort order of the children when retrieving them (default: not present)

    class Category
      <snip...>

      is :tree, :order => [:updated_at, :name]

    end

## Usage

To create the above structure, we would start with:

    the_parent = Category.create(:name => "the_parent")

### `#parent` & `#parent=`

The `#parent` instance method returns the node referenced by the foreign key - `:parent_id` or the defined `:child_key`.

    # by default #parent and #parent_id return nil when there is no parent
    the_parent.parent  # => nil

The `#parent=` instance method sets the `:parent_id` foreign key to the parent's id attribute value.

To define a parent you can use either of these syntaxes:

    a_child = Category.create(:name => "a_child", :parent => the_parent)

    a_child = Category.create(:name => "a_child", :parent_id => the_parent.id)

    a_child = Category.create(:name => "a_child")
    a_child.parent = the_parent

When retrieving the parent, you will receive the full parent object ( or `nil` if none was declared )

    a_child.parent  # => the_parent

### `#children` & `#children=`

The `#children` instance method returns all nodes with the current node as their parent, in the order specified by the `:order` configuration option.

    # by default #children return an empty Array when there are no children
    a_child.children # => []

    # or an Array with child objects when there are children
    the_parent.children # => [a_child]

The `#children=` instance method adds children by setting the `:parent_id` foreign key to the parent's id attribute value.

To add a child you can use either of these syntaxes:

    child = the_parent.children.create(:name => "child")

    grandchild1 = Category.create(:name => "grandchild1")
    child.children << grandchild1

    grandchild2 = child.children.create(:name => "grandchild2")

When retrieving children, or a child, you will receive an Array of child objects.

    child.children # => [grandchild1, grandchild2,...]

    # just retrieve the first child
    child.children.first # => grandchild1

### `#siblings`

The `#siblings` instance method returns all the children of the parent, excluding the current node.

    # by default #siblings return an empty Array when there are no siblings
    the_parent.siblings # => []

    grandchild1.siblings # => [grandchild2]

### `#generation`

The `#generation` instance method returns all the children of the parent, including the current node.

    # by default #generation return an Array with itself only when it's a 'lonely child'
    the_parent.generation # => [the_parent]

    #
    grandchild1.generation # => [grandchild1, grandchild2]

### `#ancestors`

The `#ancestors` instance method returns all the ancestors of the current node.

    # by default it returns an empty Array when born through immaculate conception (is root)
    the_parent.ancestors # => []

    grandchild2.ancestors # => [the_parent, a_child]

### `#root`

The `#root` instance method returns the root (parent) of the current node.

    # by default returns itself only when it's the root node
    the_parent.root # =>  the_parent

    a_child.root # => the_parent

    grandchild2.root # => the_parent

### `self.#first_root`

The `#first_root` class method returns the first root declared in the model.

    Category.first_root # =>  the_parent

### `self.#roots`

The `#roots` class method returns an Array of the roots declared in the model.

    Category.roots # =>  [the_parent]

    parent2 = Category.create(:name => 'parent2')

    Category.roots # =>  [the_parent, parent2]

### Summary

    parent = Category.create(:name => "parent")

    child = parent.children.create(:name => "child")

    grandchild1 = child.children.create(:name => "grandchild1")

    grandchild2 = Category.create(:name => "grandchild2")
    child.children << grandchild2

    grandchild3 = Category.create(:name => "grandchild2")
    grandchild3.parent = child

    parent.parent  # => nil
    child.parent  # => parent

    parent.children  # => [child]
    parent.children.first.children.first  # => grandchild1

    parent.siblings # => []
    grandchild1.siblings # => [grandchild2]

    parent.generation # => [parent]
    grandchild1.generation # => [grandchild1, grandchild2]

    parent.ancestors # => []
    grandchild2.ancestors # => [parent, child]

    parent.root # =>  parent
    parent.root # => parent
    grandchild2.root # => parent

    Category.first_root  # => parent
    Category.roots  # => [parent]

## Gotchas

Now there are some gotcha's that might not be entirely obvious to everyone, so let's clarify them here.

### Prevent a node being made a child of it self

By default dm-is-tree allows you to save a record as a child of it self, which is quite unnatural. To prevent this, I would humbly suggest adding this custom validation code to your model(s).

    class Category
      <snip...>

      # prevent saving Category as child of self, except when new?
      validates_with_method :parent_id,
                            :method => :category_cannot_be_made_a_child_of_self,
                            :unless => :new?

      protected

        def category_cannot_be_made_a_child_of_self
          if self.id === self.parent_id
            return [
              false,
              "A Category [ #{self.name} ] cannot be made a child of it self [ #{self.name} ]"
            ]
          else
            return true
          end
        end

    end

An example:

    parent = Category.create(:name => "parent")
    child = parent.children.create(:name => 'child')

    child.parent = child

    child.save  # => return false

    child.errors.on(:parent_id)
      # => ["A Category [ child ] cannot be made a child of it self [ child ]"]

### Sorting order within nodes

By default the sorting order is alphabetic, but this spans the entire table (with all nodes), which might not be what you want.

To prevent this, order the results by `:parent_id` first, and secondly by `:name` or whatever you wish to sort by.

    class Category
      <snip...>

      is :tree, :order => [:parent_id, :name]

    end

That's about it.

## Errors / Bugs

If something is not behaving intuitively, it is a bug, and should be reported. Report it here: datamapper.lighthouseapp.com/

## TODOs

- Make it automatically prevent saving self as child of self.

- Anything else missing?

## Note on Patches/Pull Requests

- Fork the project.

- Make your feature addition or bug fix.

- Add tests for it. This is important so we don't break it in a future version unintentionally.

- Commit, do not mess with rakefile, version, or history.

  - (if you want to have your own version, that is fine but bump version in a commit by itself we can ignore when we pull)

- Send us a pull request. Bonus points for topic branches.

## Copyright

Copyright (c) 2011 Timothy Bennett. Released under the MIT License.

See LICENSE for details.

### Credits

Credit also goes to these contributors.

Current Maintainer: Garrett Heaver (www.linkedin.com/pub/dir/garrett/heaver)

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