### Home Classes Methods

### In Files

- lib/sequel/plugins/rcte_tree.rb

### Methods

- ::apply

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# Sequel::Plugins::RcteTree

# Overview

The rcte_tree plugin deals with tree structured data stored in the database using the adjacency list model (where child rows have a foreign key pointing to the parent rows), using recursive common table expressions to load all ancestors in a single query, all descendants in a single query, and all descendants to a given level (where level 1 is children, level 2 is children and grandchildren etc.) in a single query.

# Background

There are two types of common models for storing tree structured data in an SQL database, the adjacency list model and the nested set model. Before recursive common table expressions (or similar capabilities such as CONNECT BY for Oracle), the nested set model was the only easy way to retrieve all ancestors and descendants in a single query. However, it has significant performance corner cases.

On PostgreSQL 8.4, with a significant number of rows, the nested set model is almost 500 times slower than using a recursive common table expression with the adjacency list model to get all descendants, and almost 24,000 times slower to get all descendants to a given level.

Considering that the nested set model requires more difficult management than the adjacency list model, it’s almost always better to use the adjacency list model if your database supports common table expressions. See explainextended.com/2009/09/24/adjacency-list-vs-nested-sets-postgresql/ for detailed analysis.

# Usage

The rcte_tree plugin is unlike most plugins in that it doesn’t add any class, instance, or dataset modules. It only has a single apply method, which adds four associations to the model: parent, children, ancestors, and descendants. Both the parent and children are fairly standard many_to_one and one_to_many associations, respectively. However, the ancestors and descendants associations are special. Both the ancestors and descendants associations will automatically set the parent and children associations, respectively, for current object and all of the ancestor or descendant objects, whenever they are loaded (either eagerly or lazily). Additionally, the descendants association can take a level argument when called eagerly, which limits the returned objects to only that many levels in the tree (see the Overview).

    Model.plugin :rcte_tree

    # Lazy loading
    model = Model.first
    model.parent
    model.children
    model.ancestors # Populates :parent association for all ancestors
    model.descendants # Populates :children association for all descendants

    # Eager loading - also populates the :parent and children associations
    # for all ancestors and descendants
    Model.filter(:id=>[1, 2]).eager(:ancestors, :descendants).all

    # Eager loading children and grand children
    Model.filter(:id=>[1, 2]).eager(:descendants=>2).all
    # Eager loading children, grand children, and great grand children
    Model.filter(:id=>[1, 2]).eager(:descendants=>3).all

# Options

You can override the options for any specific association by making sure the plugin options contain one of the following keys:

- :parent - hash of options for the parent association

- :children - hash of options for the children association

- :ancestors - hash of options for the ancestors association

- :descendants - hash of options for the descendants association

Note that you can change the name of the above associations by specifying a :name key in the appropriate hash of options above. For example:

    Model.plugin :rcte_tree, :parent=>{:name=>:mother},
     :children=>{:name=>:daughters}, :descendants=>{:name=>:offspring}

Any other keys in the main options hash are treated as options shared by all of the associations. Here’s a few options that affect the plugin:

- :key - The foreign key in the table that points to the primary key of the parent (default: :parent_id)

- :primary_key - The primary key to use (default: the model’s primary key)

- :key_alias - The symbol identifier to use for aliasing when eager loading (default: :x_root_x)

- :cte_name - The symbol identifier to use for the common table expression (default: :t)

- :level_alias - The symbol identifier to use when eagerly loading descendants up to a given level (default: :x_level_x)

### Public Class Methods

apply(model, opts={}) click to toggle source

Create the appropriate parent, children, ancestors, and descendants associations for the model.

    # File lib/sequel/plugins/rcte_tree.rb, line 97
    def self.apply(model, opts={})
      opts = opts.dup
      opts[:class] = model

      key = opts[:key] ||= :parent_id
      prkey = opts[:primary_key] ||= model.primary_key

      par = opts.merge(opts.fetch(:parent, {}))
      parent = par.fetch(:name, :parent)
      model.many_to_one parent, par

      chi = opts.merge(opts.fetch(:children, {}))
      childrena = chi.fetch(:name, :children)
      model.one_to_many childrena, chi

      ka = opts[:key_alias] ||= :x_root_x
      t = opts[:cte_name] ||= :t
      opts[:reciprocal] = nil
      c_all = SQL::ColumnAll.new(model.table_name)

      a = opts.merge(opts.fetch(:ancestors, {}))
      ancestors = a.fetch(:name, :ancestors)
      a[:read_only] = true unless a.has_key?(:read_only)
      a[:eager_loader_key] = key
      a[:dataset] ||= proc do
        model.from(t).
         with_recursive(t, model.filter(prkey=>send(key)),
          model.join(t, key=>prkey).
          select(c_all))
      end
      aal = Array(a[:after_load])
      aal << proc do |m, ancs|
        unless m.associations.has_key?(parent)
          parent_map = {m[prkey]=>m}
          child_map = {}
          child_map[m[key]] = m if m[key]
          m.associations[parent] = nil
          ancs.each do |obj|
            obj.associations[parent] = nil
            parent_map[obj[prkey]] = obj
            if ok = obj[key]
              child_map[ok] = obj
            end
          end
          parent_map.each do |parent_id, obj|
            if child = child_map[parent_id]
              child.associations[parent] = obj
            end
          end
        end
      end
      a[:after_load] ||= aal
      a[:eager_loader] ||= proc do |eo|
        id_map = eo[:key_hash][key]
        parent_map = {}
        children_map = {}
        eo[:rows].each do |obj|
          parent_map[obj[prkey]] = obj
          (children_map[obj[key]] ||= []) << obj
          obj.associations[ancestors] = []
          obj.associations[parent] = nil
        end
        r = model.association_reflection(ancestors)
        model.eager_loading_dataset(r,
         model.from(t).
          with_recursive(t, model.filter(prkey=>id_map.keys).
            select(SQL::AliasedExpression.new(prkey, ka), c_all),
           model.join(t, key=>prkey).
           select(SQL::QualifiedIdentifier.new(t, ka), c_all)),
         r.select,
         eo[:associations], eo).all do |obj|
          opk = obj[prkey]
          if in_pm = parent_map.has_key?(opk)
            if idm_obj = parent_map[opk]
              idm_obj.values[ka] = obj.values[ka]
              obj = idm_obj
            end
          else
            obj.associations[parent] = nil
            parent_map[opk] = obj
            (children_map[obj[key]] ||= []) << obj
          end

          if roots = id_map[obj.values.delete(ka)]
            roots.each do |root|
              root.associations[ancestors] << obj
            end
          end
        end
        parent_map.each do |parent_id, obj|
          if children = children_map[parent_id]
            children.each do |child|
              child.associations[parent] = obj
            end
          end
        end
      end
      model.one_to_many ancestors, a

      d = opts.merge(opts.fetch(:descendants, {}))
      descendants = d.fetch(:name, :descendants)
      d[:read_only] = true unless d.has_key?(:read_only)
      la = d[:level_alias] ||= :x_level_x
      d[:dataset] ||= proc do
        model.from(t).
         with_recursive(t, model.filter(key=>send(prkey)),
          model.join(t, prkey=>key).
          select(SQL::ColumnAll.new(model.table_name)))
        end
      dal = Array(d[:after_load])
      dal << proc do |m, descs|
        unless m.associations.has_key?(childrena)
          parent_map = {m[prkey]=>m}
          children_map = {}
          m.associations[childrena] = []
          descs.each do |obj|
            obj.associations[childrena] = []
            if opk = obj[prkey]
              parent_map[opk] = obj
            end
            if ok = obj[key]
              (children_map[ok] ||= []) << obj
            end
          end
          children_map.each do |parent_id, objs|
            parent_map[parent_id].associations[childrena] = objs
          end
        end
      end
      d[:after_load] = dal
      d[:eager_loader] ||= proc do |eo|
        id_map = eo[:key_hash][prkey]
        associations = eo[:associations]
        parent_map = {}
        children_map = {}
        eo[:rows].each do |obj|
          parent_map[obj[prkey]] = obj
          obj.associations[descendants] = []
          obj.associations[childrena] = []
        end
        r = model.association_reflection(descendants)
        base_case = model.filter(key=>id_map.keys).
         select(SQL::AliasedExpression.new(key, ka), c_all)
        recursive_case = model.join(t, prkey=>key).
         select(SQL::QualifiedIdentifier.new(t, ka), c_all)
        if associations.is_a?(Integer)
          level = associations
          no_cache_level = level - 1
          associations = {}
          base_case = base_case.select_more(SQL::AliasedExpression.new(0, la))
          recursive_case = recursive_case.select_more(SQL::AliasedExpression.new(SQL::QualifiedIdentifier.new(t, la) + 1, la)).filter(SQL::QualifiedIdentifier.new(t, la) < level - 1)
        end
        model.eager_loading_dataset(r,
         model.from(t).with_recursive(t, base_case, recursive_case),
         r.select,
         associations, eo).all do |obj|
          if level
            no_cache = no_cache_level == obj.values.delete(la)
          end

          opk = obj[prkey]
          if in_pm = parent_map.has_key?(opk)
            if idm_obj = parent_map[opk]
              idm_obj.values[ka] = obj.values[ka]
              obj = idm_obj
            end
          else
            obj.associations[childrena] = [] unless no_cache
            parent_map[opk] = obj
          end

          if root = id_map[obj.values.delete(ka)].first
            root.associations[descendants] << obj
          end

          (children_map[obj[key]] ||= []) << obj
        end
        children_map.each do |parent_id, objs|
          parent_map[parent_id].associations[childrena] = objs.uniq
        end
      end
      model.one_to_many descendants, d
    end

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