Module | Sequel::Model::Associations::DatasetMethods
In: | lib/sequel/model/associations.rb

Eager loading makes it so that you can load all associated records for a set of objects in a single query, instead of a separate query for each object.

Two separate implementations are provided. eager should be used most of the time, as it loads associated records using one query per association. However, it does not allow you the ability to filter based on columns in associated tables. eager_graph loads all records in one query. Using eager_graph you can filter based on columns in associated tables. However, eager_graph can be slower than eager, especially if multiple *_to_many associations are joined.

You can cascade the eager loading (loading associations’ associations) with no limit to the depth of the cascades. You do this by passing a hash to eager or eager_graph with the keys being associations of the current model and values being associations of the model associated with the current model via the key.

The arguments can be symbols or hashes with symbol keys (for cascaded eager loading). Examples:

      Album.eager(:artist).all
      Album.eager_graph(:artist).all
      Album.eager(:artist, :genre).all
      Album.eager_graph(:artist, :genre).all
      Album.eager(:artist).eager(:genre).all
      Album.eager_graph(:artist).eager(:genre).all
      Artist.eager(:albums=>:tracks).all
      Artist.eager_graph(:albums=>:tracks).all
      Artist.eager(:albums=>{:tracks=>:genre}).all
      Artist.eager_graph(:albums=>{:tracks=>:genre}).all

### Methods

eager eager_graph eager_graph_association eager_graph_associations eager_graph_build_associations eager_unique_table_alias extended

### Public Class methods

extended(obj)

Add the eager! and eager_graph! mutation methods to the dataset.

[Source]

         # File lib/sequel/model/associations.rb, line 966
    966:         def self.extended(obj)
    967:           obj.def_mutation_method(:eager, :eager_graph)
    968:         end

### Public Instance methods

eager(*associations)

The preferred eager loading method. Loads all associated records using one query for each association.

The basic idea for how it works is that the dataset is first loaded normally. Then it goes through all associations that have been specified via eager. It loads each of those associations separately, then associates them back to the original dataset via primary/foreign keys. Due to the necessity of all objects being present, you need to use .all to use eager loading, as it can‘t work with .each.

This implementation avoids the complexity of extracting an object graph out of a single dataset, by building the object graph out of multiple datasets, one for each association. By using a separate dataset for each association, it avoids problems such as aliasing conflicts and creating cartesian product result sets if multiple *_to_many eager associations are requested.

One limitation of using this method is that you cannot filter the dataset based on values of columns in an associated table, since the associations are loaded in separate queries. To do that you need to load all associations in the same query, and extract an object graph from the results of that query. If you need to filter based on columns in associated tables, look at eager_graph or join the tables you need to filter on manually.

Each association‘s order, if defined, is respected. Eager also works on a limited dataset, but does not use any :limit options for associations. If the association uses a block or has an :eager_block argument, it is used.

[Source]

          # File lib/sequel/model/associations.rb, line 996
     996:         def eager(*associations)
     997:           opt = @opts[:eager]
     998:           opt = opt ? opt.dup : {}
     999:           associations.flatten.each do |association|
    1000:             case association
    1001:               when Symbol
    1002:                 check_association(model, association)
    1003:                 opt[association] = nil
    1004:               when Hash
    1005:                 association.keys.each{|assoc| check_association(model, assoc)}
    1006:                 opt.merge!(association)
    1007:               else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    1008:             end
    1009:           end
    1010:           clone(:eager=>opt)
    1011:         end

eager_graph(*associations)

The secondary eager loading method. Loads all associations in a single query. This method should only be used if you need to filter based on columns in associated tables.

This method builds an object graph using Dataset#graph. Then it uses the graph to build the associations, and finally replaces the graph with a simple array of model objects.

Be very careful when using this with multiple *_to_many associations, as you can create large cartesian products. If you must graph multiple *_to_many associations, make sure your filters are specific if you have a large database.

Each association‘s order, if definied, is respected. eager_graph probably won‘t work correctly on a limited dataset, unless you are only graphing many_to_one associations.

Does not use the block defined for the association, since it does a single query for all objects. You can use the :graph_* association options to modify the SQL query.

Like eager, you need to call .all on the dataset for the eager loading to work. If you just call each, you will get a normal graphed result back (a hash with model object values).

[Source]

          # File lib/sequel/model/associations.rb, line 1033
    1033:         def eager_graph(*associations)
    1034:           table_name = model.table_name
    1035:           ds = if @opts[:eager_graph]
    1036:             self
    1037:           else
    1038:             # Each of the following have a symbol key for the table alias, with the following values:
    1039:             # :reciprocals - the reciprocal instance variable to use for this association
    1040:             # :requirements - array of requirements for this association
    1041:             # :alias_association_type_map - the type of association for this association
    1042:             # :alias_association_name_map - the name of the association for this association
    1043:             clone(:eager_graph=>{:requirements=>{}, :master=>model.table_name, :alias_association_type_map=>{}, :alias_association_name_map=>{}, :reciprocals=>{}})
    1044:           end
    1045:           ds.eager_graph_associations(ds, model, table_name, [], *associations)
    1046:         end

### Protected Instance methods

eager_graph_association(ds, model, ta, requirements, r, *associations)

Call graph on the association with the correct arguments, update the eager_graph data structure, and recurse into eager_graph_associations if there are any passed in associations (which would be dependencies of the current association)

Arguments:

- ds - Current dataset
- model - Current Model
- ta - table_alias used for the parent association
- requirements - an array, used as a stack for requirements
- r - association reflection for the current association
- *associations - any associations dependent on this one

[Source]

          # File lib/sequel/model/associations.rb, line 1062
    1062:         def eager_graph_association(ds, model, ta, requirements, r, *associations)
    1063:           klass = r.associated_class
    1064:           assoc_name = r[:name]
    1065:           assoc_table_alias = ds.eager_unique_table_alias(ds, assoc_name)
    1066:           ds = r[:eager_grapher].call(ds, assoc_table_alias, ta)
    1067:           ds = ds.order_more(*Array(r[:order]).map{|c| eager_graph_qualify_order(assoc_table_alias, c)}) if r[:order] and r[:order_eager_graph]
    1068:           eager_graph = ds.opts[:eager_graph]
    1069:           eager_graph[:requirements][assoc_table_alias] = requirements.dup
    1070:           eager_graph[:alias_association_name_map][assoc_table_alias] = assoc_name
    1071:           eager_graph[:alias_association_type_map][assoc_table_alias] = r.returns_array?
    1072:           ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty?
    1073:           ds
    1074:         end

eager_graph_associations(ds, model, ta, requirements, *associations)

Check the associations are valid for the given model. Call eager_graph_association on each association.

Arguments:

- ds - Current dataset
- model - Current Model
- ta - table_alias used for the parent association
- requirements - an array, used as a stack for requirements
- *associations - the associations to add to the graph

[Source]

          # File lib/sequel/model/associations.rb, line 1085
    1085:         def eager_graph_associations(ds, model, ta, requirements, *associations)
    1086:           return ds if associations.empty?
    1087:           associations.flatten.each do |association|
    1088:             ds = case association
    1089:             when Symbol
    1090:               ds.eager_graph_association(ds, model, ta, requirements, check_association(model, association))
    1091:             when Hash
    1092:               association.each do |assoc, assoc_assocs|
    1093:                 ds = ds.eager_graph_association(ds, model, ta, requirements, check_association(model, assoc), assoc_assocs)
    1094:               end
    1095:               ds
    1096:             else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    1097:             end
    1098:           end
    1099:           ds
    1100:         end

eager_graph_build_associations(record_graphs)

Build associations out of the array of returned object graphs.

[Source]

          # File lib/sequel/model/associations.rb, line 1103
    1103:         def eager_graph_build_associations(record_graphs)
    1104:           eager_graph = @opts[:eager_graph]
    1105:           master = eager_graph[:master]
    1106:           requirements = eager_graph[:requirements]
    1107:           alias_map = eager_graph[:alias_association_name_map]
    1108:           type_map = eager_graph[:alias_association_type_map]
    1109:           reciprocal_map = eager_graph[:reciprocals]
    1110:
    1111:           # Make dependency map hash out of requirements array for each association.
    1112:           # This builds a tree of dependencies that will be used for recursion
    1113:           # to ensure that all parts of the object graph are loaded into the
    1114:           # appropriate subordinate association.
    1115:           dependency_map = {}
    1116:           # Sort the associations by requirements length, so that
    1117:           # requirements are added to the dependency hash before their
    1118:           # dependencies.
    1119:           requirements.sort_by{|a| a[1].length}.each do |ta, deps|
    1120:             if deps.empty?
    1121:               dependency_map[ta] = {}
    1122:             else
    1123:               deps = deps.dup
    1124:               hash = dependency_map[deps.shift]
    1125:               deps.each do |dep|
    1126:                 hash = hash[dep]
    1127:               end
    1128:               hash[ta] = {}
    1129:             end
    1130:           end
    1131:
    1132:           # This mapping is used to make sure that duplicate entries in the
    1133:           # result set are mapped to a single record.  For example, using a
    1134:           # single one_to_many association with 10 associated records,
    1135:           # the main object will appear in the object graph 10 times.
    1136:           # We map by primary key, if available, or by the object's entire values,
    1137:           # if not. The mapping must be per table, so create sub maps for each table
    1138:           # alias.
    1139:           records_map = {master=>{}}
    1140:           alias_map.keys.each{|ta| records_map[ta] = {}}
    1141:
    1142:           # This will hold the final record set that we will be replacing the object graph with.
    1143:           records = []
    1144:           record_graphs.each do |record_graph|
    1145:             primary_record = record_graph[master]
    1146:             key = primary_record.pk || primary_record.values.sort_by{|x| x[0].to_s}
    1147:             if cached_pr = records_map[master][key]
    1148:               primary_record = cached_pr
    1149:             else
    1150:               records_map[master][key] = primary_record
    1151:               # Only add it to the list of records to return if it is a new record
    1152:               records.push(primary_record)
    1153:             end
    1154:             # Build all associations for the current object and it's dependencies
    1155:             eager_graph_build_associations_graph(dependency_map, alias_map, type_map, reciprocal_map, records_map, primary_record, record_graph)
    1156:           end
    1157:
    1158:           # Remove duplicate records from all associations if this graph could possibly be a cartesian product
    1159:           eager_graph_make_associations_unique(records, dependency_map, alias_map, type_map) if type_map.values.select{|v| v}.length > 1
    1160:
    1161:           # Replace the array of object graphs with an array of model objects
    1162:           record_graphs.replace(records)
    1163:         end

eager_unique_table_alias(ds, table_alias)

Creates a unique table alias that hasn‘t already been used in the query. Will either be the table_alias itself or table_alias_N for some integer N (starting at 0 and increasing until an unused one is found).

[Source]

          # File lib/sequel/model/associations.rb, line 1168
    1168:         def eager_unique_table_alias(ds, table_alias)
    1169:           used_aliases = ds.opts[:from]
    1170:           graph = ds.opts[:graph]
    1171:           used_aliases += graph[:table_aliases].keys if graph
    1172:           if used_aliases.include?(table_alias)
    1173:             i = 0
    1174:             loop do
    1175:               ta = "#{table_alias}_#{i}""#{table_alias}_#{i}"
    1176:               return ta unless used_aliases.include?(ta)
    1177:               i += 1
    1178:             end
    1179:           end
    1180:           table_alias
    1181:         end

[Validate]