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 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)

Methods

Public Class

apply

Public Class methods

apply (model, opts=OPTS)

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

[show source]

# File lib/sequel/plugins/rcte_tree.rb, line 95
def self.apply(model, opts=OPTS)
  model.plugin :tree, opts

  opts = opts.dup
  opts[:class] = model
  opts[:methods_module] = Module.new
  model.send(:include, opts[:methods_module])
  
  key = opts[:key] ||= :parent_id
  prkey = opts[:primary_key] ||= model.primary_key
  ka = opts[:key_alias] ||= :x_root_x
  t = opts[:cte_name] ||= :t
  c_all = if model.dataset.recursive_cte_requires_column_aliases?
    # Work around Oracle/ruby-oci8 bug that returns integers as BigDecimals in recursive queries.
    conv_bd = model.db.database_type == :oracle
    col_aliases = model.dataset.columns
    model_table = model.table_name
    col_aliases.map{|c| SQL::QualifiedIdentifier.new(model_table, c)}
  else
    [SQL::ColumnAll.new(model.table_name)]
  end
  
  bd_conv = lambda{|v| conv_bd && v.is_a?(BigDecimal) ? v.to_i : v}

  key_array = Array(key)
  prkey_array = Array(prkey)
  if key.is_a?(Array)
    key_conv = lambda{|m| key_array.map{|k| m[k]}}
    key_present = lambda{|m| key_conv[m].all?}
    prkey_conv = lambda{|m| prkey_array.map{|k| m[k]}}
    key_aliases = (0...key_array.length).map{|i| :"#{ka}_#{i}"}
    ka_conv = lambda{|m| key_aliases.map{|k| m[k]}}
    ancestor_base_case_columns = prkey_array.zip(key_aliases).map{|k, ka_| SQL::AliasedExpression.new(k, ka_)} + c_all
    descendant_base_case_columns = key_array.zip(key_aliases).map{|k, ka_| SQL::AliasedExpression.new(k, ka_)} + c_all
    recursive_case_columns = prkey_array.zip(key_aliases).map{|k, ka_| SQL::QualifiedIdentifier.new(t, ka_)} + c_all
    extract_key_alias = lambda{|m| key_aliases.map{|ka_| bd_conv[m.values.delete(ka_)]}}
  else
    key_present = key_conv = lambda{|m| m[key]}
    prkey_conv = lambda{|m| m[prkey]}
    key_aliases = [ka]
    ka_conv = lambda{|m| m[ka]}
    ancestor_base_case_columns = [SQL::AliasedExpression.new(prkey, ka)] + c_all
    descendant_base_case_columns = [SQL::AliasedExpression.new(key, ka)] + c_all
    recursive_case_columns = [SQL::QualifiedIdentifier.new(t, ka)] + c_all
    extract_key_alias = lambda{|m| bd_conv[m.values.delete(ka)]}
  end
  
  parent = opts.merge(opts.fetch(:parent, {})).fetch(:name, :parent)
  childrena = opts.merge(opts.fetch(:children, {})).fetch(:name, :children)
  
  opts[:reciprocal] = nil
  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
    base_ds = model.filter(prkey_array.zip(key_array.map{|k| send(k)}))
    recursive_ds = model.join(t, key_array.zip(prkey_array))
    if c = a[:conditions]
      (base_ds, recursive_ds) = [base_ds, recursive_ds].collect do |ds|
        (c.is_a?(Array) && !Sequel.condition_specifier?(c)) ? ds.filter(*c) : ds.filter(c)
      end
    end
    table_alias = model.dataset.schema_and_table(model.table_name)[1].to_sym
    model.from(SQL::AliasedExpression.new(t, table_alias)).
     with_recursive(t, col_aliases ? base_ds.select(*col_aliases) : base_ds.select_all,
      recursive_ds.select(*c_all),
      :args=>col_aliases)
  end
  aal = Array(a[:after_load])
  aal << proc do |m, ancs|
    unless m.associations.has_key?(parent)
      parent_map = {prkey_conv[m]=>m}
      child_map = {}
      child_map[key_conv[m]] = m if key_present[m]
      m.associations[parent] = nil
      ancs.each do |obj|
        obj.associations[parent] = nil
        parent_map[prkey_conv[obj]] = obj
        if ok = key_conv[obj]
          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[:id_map]
    parent_map = {}
    children_map = {}
    eo[:rows].each do |obj|
      parent_map[prkey_conv[obj]] = obj
      (children_map[key_conv[obj]] ||= []) << obj
      obj.associations[ancestors] = []
      obj.associations[parent] = nil
    end
    r = model.association_reflection(ancestors)
    base_case = model.filter(prkey=>id_map.keys).
     select(*ancestor_base_case_columns)
    recursive_case = model.join(t, key_array.zip(prkey_array)).
     select(*recursive_case_columns)
    if c = r[:conditions]
      (base_case, recursive_case) = [base_case, recursive_case].collect do |ds|
        (c.is_a?(Array) && !Sequel.condition_specifier?(c)) ? ds.filter(*c) : ds.filter(c)
      end
    end
    table_alias = model.dataset.schema_and_table(model.table_name)[1].to_sym
    elds = model.eager_loading_dataset(r,
     model.from(SQL::AliasedExpression.new(t, table_alias)).
      with_recursive(t, base_case,
       recursive_case,
       :args=>((key_aliases + col_aliases) if col_aliases)),
     r.select,
     eo[:associations], eo)
    elds = elds.select_append(ka) unless elds.opts[:select] == nil
    elds.all do |obj|
      opk = prkey_conv[obj]
      if parent_map.has_key?(opk)
        if idm_obj = parent_map[opk]
          key_aliases.each{|ka_| idm_obj.values[ka_] = obj.values[ka_]}
          obj = idm_obj
        end
      else
        obj.associations[parent] = nil
        parent_map[opk] = obj
        (children_map[key_conv[obj]] ||= []) << obj
      end
      
      if roots = id_map[extract_key_alias[obj]]
        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
    base_ds = model.filter(key_array.zip(prkey_array.map{|k| send(k)}))
    recursive_ds = model.join(t, prkey_array.zip(key_array))
    if c = d[:conditions]
      (base_ds, recursive_ds) = [base_ds, recursive_ds].collect do |ds|
        (c.is_a?(Array) && !Sequel.condition_specifier?(c)) ? ds.filter(*c) : ds.filter(c)
      end
    end
    table_alias = model.dataset.schema_and_table(model.table_name)[1].to_sym
    model.from(SQL::AliasedExpression.new(t, table_alias)).
     with_recursive(t, col_aliases ? base_ds.select(*col_aliases) : base_ds.select_all,
      recursive_ds.select(*c_all),
      :args=>col_aliases)
    end
  dal = Array(d[:after_load])
  dal << proc do |m, descs|
    unless m.associations.has_key?(childrena)
      parent_map = {prkey_conv[m]=>m}
      children_map = {}
      m.associations[childrena] = []
      descs.each do |obj|
        obj.associations[childrena] = []
        if opk = prkey_conv[obj]
          parent_map[opk] = obj
        end
        if ok = key_conv[obj]
          (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[:id_map]
    associations = eo[:associations]
    parent_map = {}
    children_map = {}
    eo[:rows].each do |obj|
      parent_map[prkey_conv[obj]] = obj
      obj.associations[descendants] = []
      obj.associations[childrena] = []
    end
    r = model.association_reflection(descendants)
    base_case = model.filter(key=>id_map.keys).
     select(*descendant_base_case_columns)
    recursive_case = model.join(t, prkey_array.zip(key_array)).
     select(*recursive_case_columns)
    if c = r[:conditions]
      (base_case, recursive_case) = [base_case, recursive_case].collect do |ds|
        (c.is_a?(Array) && !Sequel.condition_specifier?(c)) ? ds.filter(*c) : ds.filter(c)
      end
    end
    if associations.is_a?(Integer)
      level = associations
      no_cache_level = level - 1
      associations = {}
      base_case = base_case.select_more(SQL::AliasedExpression.new(Sequel.cast(0, Integer), 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
    table_alias = model.dataset.schema_and_table(model.table_name)[1].to_sym
    elds = model.eager_loading_dataset(r,
     model.from(SQL::AliasedExpression.new(t, table_alias)).with_recursive(t, base_case, recursive_case,
      :args=>((key_aliases + col_aliases + (level ? [la] : [])) if col_aliases)),
     r.select,
     associations, eo)
    elds = elds.select_append(ka) unless elds.opts[:select] == nil
    elds.all do |obj|
      if level
        no_cache = no_cache_level == obj.values.delete(la)
      end
      
      opk = prkey_conv[obj]
      if parent_map.has_key?(opk)
        if idm_obj = parent_map[opk]
          key_aliases.each{|ka_| 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[extract_key_alias[obj]].first
        root.associations[descendants] << obj
      end
      
      (children_map[key_conv[obj]] ||= []) << 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