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
Add the eager! and eager_graph! mutation methods to the dataset.
# File lib/sequel/model/associations.rb, line 1030 1030: def self.extended(obj) 1031: obj.def_mutation_method(:eager, :eager_graph) 1032: end
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.
# File lib/sequel/model/associations.rb, line 1060 1060: def eager(*associations) 1061: opt = @opts[:eager] 1062: opt = opt ? opt.dup : {} 1063: associations.flatten.each do |association| 1064: case association 1065: when Symbol 1066: check_association(model, association) 1067: opt[association] = nil 1068: when Hash 1069: association.keys.each{|assoc| check_association(model, assoc)} 1070: opt.merge!(association) 1071: else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 1072: end 1073: end 1074: clone(:eager=>opt) 1075: end
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).
# File lib/sequel/model/associations.rb, line 1097 1097: def eager_graph(*associations) 1098: table_name = model.table_name 1099: ds = if @opts[:eager_graph] 1100: self 1101: else 1102: # Each of the following have a symbol key for the table alias, with the following values: 1103: # :reciprocals - the reciprocal instance variable to use for this association 1104: # :requirements - array of requirements for this association 1105: # :alias_association_type_map - the type of association for this association 1106: # :alias_association_name_map - the name of the association for this association 1107: clone(:eager_graph=>{:requirements=>{}, :master=>model.table_name, :alias_association_type_map=>{}, :alias_association_name_map=>{}, :reciprocals=>{}, :cartesian_product_number=>0}) 1108: end 1109: ds.eager_graph_associations(ds, model, table_name, [], *associations) 1110: end
Do not attempt to split the result set into associations, just return results as simple objects. This is useful if you want to use eager_graph as a shortcut to have all of the joins and aliasing set up, but want to do something else with the dataset.
# File lib/sequel/model/associations.rb, line 1116 1116: def ungraphed 1117: super.clone(:eager_graph=>nil) 1118: end
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:
# File lib/sequel/model/associations.rb, line 1134 1134: def eager_graph_association(ds, model, ta, requirements, r, *associations) 1135: klass = r.associated_class 1136: assoc_name = r[:name] 1137: assoc_table_alias = ds.eager_unique_table_alias(ds, assoc_name) 1138: ds = r[:eager_grapher].call(ds, assoc_table_alias, ta) 1139: 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] 1140: eager_graph = ds.opts[:eager_graph] 1141: eager_graph[:requirements][assoc_table_alias] = requirements.dup 1142: eager_graph[:alias_association_name_map][assoc_table_alias] = assoc_name 1143: eager_graph[:alias_association_type_map][assoc_table_alias] = r.returns_array? 1144: eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2 1145: ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty? 1146: ds 1147: end
Check the associations are valid for the given model. Call eager_graph_association on each association.
Arguments:
# File lib/sequel/model/associations.rb, line 1158 1158: def eager_graph_associations(ds, model, ta, requirements, *associations) 1159: return ds if associations.empty? 1160: associations.flatten.each do |association| 1161: ds = case association 1162: when Symbol 1163: ds.eager_graph_association(ds, model, ta, requirements, check_association(model, association)) 1164: when Hash 1165: association.each do |assoc, assoc_assocs| 1166: ds = ds.eager_graph_association(ds, model, ta, requirements, check_association(model, assoc), assoc_assocs) 1167: end 1168: ds 1169: else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 1170: end 1171: end 1172: ds 1173: end
Build associations out of the array of returned object graphs.
# File lib/sequel/model/associations.rb, line 1176 1176: def eager_graph_build_associations(record_graphs) 1177: eager_graph = @opts[:eager_graph] 1178: master = eager_graph[:master] 1179: requirements = eager_graph[:requirements] 1180: alias_map = eager_graph[:alias_association_name_map] 1181: type_map = eager_graph[:alias_association_type_map] 1182: reciprocal_map = eager_graph[:reciprocals] 1183: 1184: # Make dependency map hash out of requirements array for each association. 1185: # This builds a tree of dependencies that will be used for recursion 1186: # to ensure that all parts of the object graph are loaded into the 1187: # appropriate subordinate association. 1188: dependency_map = {} 1189: # Sort the associations by requirements length, so that 1190: # requirements are added to the dependency hash before their 1191: # dependencies. 1192: requirements.sort_by{|a| a[1].length}.each do |ta, deps| 1193: if deps.empty? 1194: dependency_map[ta] = {} 1195: else 1196: deps = deps.dup 1197: hash = dependency_map[deps.shift] 1198: deps.each do |dep| 1199: hash = hash[dep] 1200: end 1201: hash[ta] = {} 1202: end 1203: end 1204: 1205: # This mapping is used to make sure that duplicate entries in the 1206: # result set are mapped to a single record. For example, using a 1207: # single one_to_many association with 10 associated records, 1208: # the main object will appear in the object graph 10 times. 1209: # We map by primary key, if available, or by the object's entire values, 1210: # if not. The mapping must be per table, so create sub maps for each table 1211: # alias. 1212: records_map = {master=>{}} 1213: alias_map.keys.each{|ta| records_map[ta] = {}} 1214: 1215: # This will hold the final record set that we will be replacing the object graph with. 1216: records = [] 1217: record_graphs.each do |record_graph| 1218: primary_record = record_graph[master] 1219: key = primary_record.pk_or_nil || primary_record.values.sort_by{|x| x[0].to_s} 1220: if cached_pr = records_map[master][key] 1221: primary_record = cached_pr 1222: else 1223: records_map[master][key] = primary_record 1224: # Only add it to the list of records to return if it is a new record 1225: records.push(primary_record) 1226: end 1227: # Build all associations for the current object and it's dependencies 1228: eager_graph_build_associations_graph(dependency_map, alias_map, type_map, reciprocal_map, records_map, primary_record, record_graph) 1229: end 1230: 1231: # Remove duplicate records from all associations if this graph could possibly be a cartesian product 1232: eager_graph_make_associations_unique(records, dependency_map, alias_map, type_map) if eager_graph[:cartesian_product_number] > 1 1233: 1234: # Replace the array of object graphs with an array of model objects 1235: record_graphs.replace(records) 1236: end
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).
# File lib/sequel/model/associations.rb, line 1241 1241: def eager_unique_table_alias(ds, table_alias) 1242: used_aliases = ds.opts[:from] 1243: used_aliases += ds.opts[:join].map{|j| j.table_alias || j.table} if ds.opts[:join] 1244: graph = ds.opts[:graph] 1245: used_aliases += graph[:table_aliases].keys if graph 1246: if used_aliases.include?(table_alias) 1247: i = 0 1248: loop do 1249: ta = "#{table_alias}_#{i}""#{table_alias}_#{i}" 1250: return ta unless used_aliases.include?(ta) 1251: i += 1 1252: end 1253: end 1254: table_alias 1255: end