Module | Sequel::Model::Associations::EagerLoading |
In: |
lib/sequel_model/eager_loading.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 much 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/eager_loading.rb, line 31 31: def self.extended(obj) 32: obj.def_mutation_method(:eager, :eager_graph) 33: 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/eager_loading.rb, line 61 61: def eager(*associations) 62: model = check_model 63: opt = @opts[:eager] 64: opt = opt ? opt.dup : {} 65: associations.flatten.each do |association| 66: case association 67: when Symbol 68: check_association(model, association) 69: opt[association] = nil 70: when Hash 71: association.keys.each{|assoc| check_association(model, assoc)} 72: opt.merge!(association) 73: else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 74: end 75: end 76: clone(:eager=>opt) 77: 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_join_type, :graph_conditions, and :graph_join_table_conditions association options to modify the SQL query.
# File lib/sequel_model/eager_loading.rb, line 97 97: def eager_graph(*associations) 98: model = check_model 99: table_name = model.table_name 100: ds = if @opts[:eager_graph] 101: self 102: else 103: # Each of the following have a symbol key for the table alias, with the following values: 104: # :reciprocals - the reciprocal instance variable to use for this association 105: # :requirements - array of requirements for this association 106: # :alias_association_type_map - the type of association for this association 107: # :alias_association_name_map - the name of the association for this association 108: clone(:eager_graph=>{:requirements=>{}, :master=>model.table_name, :alias_association_type_map=>{}, :alias_association_name_map=>{}, :reciprocals=>{}}) 109: end 110: ds.eager_graph_associations(ds, model, table_name, [], *associations) 111: 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/eager_loading.rb, line 127 127: def eager_graph_association(ds, model, ta, requirements, r, *associations) 128: klass = r.associated_class 129: assoc_name = r[:name] 130: assoc_table_alias = ds.eager_unique_table_alias(ds, assoc_name) 131: ds = r[:eager_grapher].call(ds, assoc_table_alias, ta) 132: 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] 133: eager_graph = ds.opts[:eager_graph] 134: eager_graph[:requirements][assoc_table_alias] = requirements.dup 135: eager_graph[:alias_association_name_map][assoc_table_alias] = assoc_name 136: eager_graph[:alias_association_type_map][assoc_table_alias] = r.returns_array? 137: ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty? 138: ds 139: end
Check the associations are valid for the given model. Call eager_graph_association on each association.
Arguments:
# File lib/sequel_model/eager_loading.rb, line 150 150: def eager_graph_associations(ds, model, ta, requirements, *associations) 151: return ds if associations.empty? 152: associations.flatten.each do |association| 153: ds = case association 154: when Symbol 155: ds.eager_graph_association(ds, model, ta, requirements, check_association(model, association)) 156: when Hash 157: association.each do |assoc, assoc_assocs| 158: ds = ds.eager_graph_association(ds, model, ta, requirements, check_association(model, assoc), assoc_assocs) 159: end 160: ds 161: else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 162: end 163: end 164: ds 165: end
Build associations out of the array of returned object graphs.
# File lib/sequel_model/eager_loading.rb, line 168 168: def eager_graph_build_associations(record_graphs) 169: eager_graph = @opts[:eager_graph] 170: master = eager_graph[:master] 171: requirements = eager_graph[:requirements] 172: alias_map = eager_graph[:alias_association_name_map] 173: type_map = eager_graph[:alias_association_type_map] 174: reciprocal_map = eager_graph[:reciprocals] 175: 176: # Make dependency map hash out of requirements array for each association. 177: # This builds a tree of dependencies that will be used for recursion 178: # to ensure that all parts of the object graph are loaded into the 179: # appropriate subordinate association. 180: dependency_map = {} 181: # Sort the associations be requirements length, so that 182: # requirements are added to the dependency hash before their 183: # dependencies. 184: requirements.sort_by{|a| a[1].length}.each do |ta, deps| 185: if deps.empty? 186: dependency_map[ta] = {} 187: else 188: deps = deps.dup 189: hash = dependency_map[deps.shift] 190: deps.each do |dep| 191: hash = hash[dep] 192: end 193: hash[ta] = {} 194: end 195: end 196: 197: # This mapping is used to make sure that duplicate entries in the 198: # result set are mapped to a single record. For example, using a 199: # single one_to_many association with 10 associated records, 200: # the main object will appear in the object graph 10 times. 201: # We map by primary key, if available, or by the object's entire values, 202: # if not. The mapping must be per table, so create sub maps for each table 203: # alias. 204: records_map = {master=>{}} 205: alias_map.keys.each{|ta| records_map[ta] = {}} 206: 207: # This will hold the final record set that we will be replacing the object graph with. 208: records = [] 209: record_graphs.each do |record_graph| 210: primary_record = record_graph[master] 211: key = primary_record.pk || primary_record.values.sort_by{|x| x[0].to_s} 212: if cached_pr = records_map[master][key] 213: primary_record = cached_pr 214: else 215: records_map[master][key] = primary_record 216: # Only add it to the list of records to return if it is a new record 217: records.push(primary_record) 218: end 219: # Build all associations for the current object and it's dependencies 220: eager_graph_build_associations_graph(dependency_map, alias_map, type_map, reciprocal_map, records_map, primary_record, record_graph) 221: end 222: 223: # Remove duplicate records from all associations if this graph could possibly be a cartesian product 224: eager_graph_make_associations_unique(records, dependency_map, alias_map, type_map) if type_map.values.select{|v| v}.length > 1 225: 226: # Replace the array of object graphs with an array of model objects 227: record_graphs.replace(records) 228: 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/eager_loading.rb, line 233 233: def eager_unique_table_alias(ds, table_alias) 234: used_aliases = ds.opts[:from] 235: graph = ds.opts[:graph] 236: used_aliases += graph[:table_aliases].keys if graph 237: if used_aliases.include?(table_alias) 238: i = 0 239: loop do 240: ta = "#{table_alias}_#{i}""#{table_alias}_#{i}" 241: return ta unless used_aliases.include?(ta) 242: i += 1 243: end 244: end 245: table_alias 246: end