Module Sequel::Plugins::RcteTree
In: lib/sequel/plugins/rcte_tree.rb

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)

Methods

apply  

Public Class methods

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

[Source]

     # File lib/sequel/plugins/rcte_tree.rb, line 97
 97:       def self.apply(model, opts={})
 98:         opts = opts.dup
 99:         opts[:class] = model
100:         
101:         key = opts[:key] ||= :parent_id
102:         prkey = opts[:primary_key] ||= model.primary_key
103:         
104:         par = opts.merge(opts.fetch(:parent, {}))
105:         parent = par.fetch(:name, :parent)
106:         model.many_to_one parent, par
107:         
108:         chi = opts.merge(opts.fetch(:children, {}))
109:         childrena = chi.fetch(:name, :children)
110:         model.one_to_many childrena, chi
111:         
112:         ka = opts[:key_alias] ||= :x_root_x
113:         t = opts[:cte_name] ||= :t
114:         opts[:reciprocal] = nil
115:         c_all = SQL::ColumnAll.new(model.table_name)
116:         
117:         a = opts.merge(opts.fetch(:ancestors, {}))
118:         ancestors = a.fetch(:name, :ancestors)
119:         a[:read_only] = true unless a.has_key?(:read_only)
120:         a[:eager_loader_key] = key
121:         a[:dataset] ||= proc do
122:           model.from(t).
123:            with_recursive(t, model.filter(prkey=>send(key)),
124:             model.join(t, key=>prkey).
125:             select(c_all))
126:         end
127:         aal = Array(a[:after_load])
128:         aal << proc do |m, ancs|
129:           unless m.associations.has_key?(parent)
130:             parent_map = {m[prkey]=>m}
131:             child_map = {}
132:             child_map[m[key]] = m if m[key]
133:             m.associations[parent] = nil
134:             ancs.each do |obj|
135:               obj.associations[parent] = nil
136:               parent_map[obj[prkey]] = obj
137:               if ok = obj[key]
138:                 child_map[ok] = obj
139:               end
140:             end
141:             parent_map.each do |parent_id, obj|
142:               if child = child_map[parent_id]
143:                 child.associations[parent] = obj
144:               end
145:             end
146:           end
147:         end
148:         a[:after_load] ||= aal
149:         a[:eager_loader] ||= proc do |eo|
150:           id_map = eo[:key_hash][key]
151:           parent_map = {}
152:           children_map = {}
153:           eo[:rows].each do |obj|
154:             parent_map[obj[prkey]] = obj
155:             (children_map[obj[key]] ||= []) << obj
156:             obj.associations[ancestors] = []
157:             obj.associations[parent] = nil
158:           end
159:           r = model.association_reflection(ancestors)
160:           model.eager_loading_dataset(r,
161:            model.from(t).
162:             with_recursive(t, model.filter(prkey=>id_map.keys).
163:               select(SQL::AliasedExpression.new(prkey, ka), c_all),
164:              model.join(t, key=>prkey).
165:              select(SQL::QualifiedIdentifier.new(t, ka), c_all)),
166:            r.select,
167:            eo[:associations], eo).all do |obj|
168:             opk = obj[prkey]
169:             if in_pm = parent_map.has_key?(opk)
170:               if idm_obj = parent_map[opk]
171:                 idm_obj.values[ka] = obj.values[ka]
172:                 obj = idm_obj
173:               end
174:             else
175:               obj.associations[parent] = nil
176:               parent_map[opk] = obj
177:               (children_map[obj[key]] ||= []) << obj
178:             end
179:             
180:             if roots = id_map[obj.values.delete(ka)]
181:               roots.each do |root|
182:                 root.associations[ancestors] << obj
183:               end
184:             end
185:           end
186:           parent_map.each do |parent_id, obj|
187:             if children = children_map[parent_id]
188:               children.each do |child|
189:                 child.associations[parent] = obj
190:               end
191:             end
192:           end
193:         end
194:         model.one_to_many ancestors, a
195:         
196:         d = opts.merge(opts.fetch(:descendants, {}))
197:         descendants = d.fetch(:name, :descendants)
198:         d[:read_only] = true unless d.has_key?(:read_only)
199:         la = d[:level_alias] ||= :x_level_x
200:         d[:dataset] ||= proc do
201:           model.from(t).
202:            with_recursive(t, model.filter(key=>send(prkey)),
203:             model.join(t, prkey=>key).
204:             select(SQL::ColumnAll.new(model.table_name)))
205:           end
206:         dal = Array(d[:after_load])
207:         dal << proc do |m, descs|
208:           unless m.associations.has_key?(childrena)
209:             parent_map = {m[prkey]=>m}
210:             children_map = {}
211:             m.associations[childrena] = []
212:             descs.each do |obj|
213:               obj.associations[childrena] = []
214:               if opk = obj[prkey]
215:                 parent_map[opk] = obj
216:               end
217:               if ok = obj[key]
218:                 (children_map[ok] ||= []) << obj
219:               end
220:             end
221:             children_map.each do |parent_id, objs|
222:               parent_map[parent_id].associations[childrena] = objs
223:             end
224:           end
225:         end
226:         d[:after_load] = dal
227:         d[:eager_loader] ||= proc do |eo|
228:           id_map = eo[:key_hash][prkey]
229:           associations = eo[:associations]
230:           parent_map = {}
231:           children_map = {}
232:           eo[:rows].each do |obj|
233:             parent_map[obj[prkey]] = obj
234:             obj.associations[descendants] = []
235:             obj.associations[childrena] = []
236:           end
237:           r = model.association_reflection(descendants)
238:           base_case = model.filter(key=>id_map.keys).
239:            select(SQL::AliasedExpression.new(key, ka), c_all)
240:           recursive_case = model.join(t, prkey=>key).
241:            select(SQL::QualifiedIdentifier.new(t, ka), c_all)
242:           if associations.is_a?(Integer)
243:             level = associations
244:             no_cache_level = level - 1
245:             associations = {}
246:             base_case = base_case.select_more(SQL::AliasedExpression.new(0, la))
247:             recursive_case = recursive_case.select_more(SQL::AliasedExpression.new(SQL::QualifiedIdentifier.new(t, la) + 1, la)).filter(SQL::QualifiedIdentifier.new(t, la) < level - 1)
248:           end
249:           model.eager_loading_dataset(r,
250:            model.from(t).with_recursive(t, base_case, recursive_case),
251:            r.select,
252:            associations, eo).all do |obj|
253:             if level
254:               no_cache = no_cache_level == obj.values.delete(la)
255:             end
256:             
257:             opk = obj[prkey]
258:             if in_pm = parent_map.has_key?(opk)
259:               if idm_obj = parent_map[opk]
260:                 idm_obj.values[ka] = obj.values[ka]
261:                 obj = idm_obj
262:               end
263:             else
264:               obj.associations[childrena] = [] unless no_cache
265:               parent_map[opk] = obj
266:             end
267:             
268:             if root = id_map[obj.values.delete(ka)].first
269:               root.associations[descendants] << obj
270:             end
271:             
272:             (children_map[obj[key]] ||= []) << obj
273:           end
274:           children_map.each do |parent_id, objs|
275:             parent_map[parent_id].associations[childrena] = objs.uniq
276:           end
277:         end
278:         model.one_to_many descendants, d
279:       end

[Validate]