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 or order based on columns in associated tables. eager_graph loads all records in a single query using JOINs, allowing you to filter or order based on columns in associated tables. However, eager_graph can be slower than eager, especially if multiple one_to_many or many_to_many associations are joined.

You can cascade the eager loading (loading associations on associated objects) 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

You can also pass a callback as a hash value in order to customize the dataset being eager loaded at query time, analogous to the way the :eager_block association option allows you to customize it at association definition time. For example, if you wanted artists with their albums since 1990:

  Artist.eager(:albums => proc{|ds| ds.filter{year > 1990}})

Or if you needed albums and their artist‘s name only, using a single query:

  Albums.eager_graph(:artist => proc{|ds| ds.select(:name)})

To cascade eager loading while using a callback, you substitute the cascaded associations with a single entry hash that has the proc callback as the key and the cascaded associations as the value. This will load artists with their albums since 1990, and also the tracks on those albums and the genre for those tracks:

  Artist.eager(:albums => {proc{|ds| ds.filter{year > 1990}}=>{:tracks => :genre}})

Methods

Public Class methods

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

[Source]

      # File lib/sequel/model/associations.rb, line 1331
1331:         def self.extended(obj)
1332:           obj.def_mutation_method(:eager, :eager_graph)
1333:         end

Public Instance methods

If the expression is in the form x = y where y is a Sequel::Model instance, assume x is an association symbol and look up the association reflection via the dataset‘s model. From there, return the appropriate SQL based on the type of association and the values of the foreign/primary keys of y. For most association types, this is a simple transformation, but for many_to_many associations this creates a subquery to the join table.

[Source]

      # File lib/sequel/model/associations.rb, line 1419
1419:         def complex_expression_sql(op, args)
1420:           r = args.at(1)
1421:           if (((op == '=''=' || op == '!=''!=') and r.is_a?(Sequel::Model)) ||
1422:               (multiple = ((op == :IN || op == 'NOT IN''NOT IN') and ((is_ds = r.is_a?(Sequel::Dataset)) or r.all?{|x| x.is_a?(Sequel::Model)}))))
1423:             l = args.at(0)
1424:             if ar = model.association_reflections[l]
1425:               if multiple
1426:                 klass = ar.associated_class
1427:                 if is_ds
1428:                   if r.respond_to?(:model)
1429:                     unless r.model <= klass
1430:                       # A dataset for a different model class, could be a valid regular query
1431:                       return super
1432:                     end
1433:                   else
1434:                     # Not a model dataset, could be a valid regular query
1435:                     return super
1436:                   end
1437:                 else
1438:                   unless r.all?{|x| x.is_a?(klass)}
1439:                     raise Sequel::Error, "invalid association class for one object for association #{l.inspect} used in dataset filter for model #{model.inspect}, expected class #{klass.inspect}"
1440:                   end
1441:                 end
1442:               elsif !r.is_a?(ar.associated_class)
1443:                 raise Sequel::Error, "invalid association class #{r.class.inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}, expected class #{ar.associated_class.inspect}"
1444:               end
1445: 
1446:               if exp = association_filter_expression(op, ar, r)
1447:                 literal(exp)
1448:               else
1449:                 raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}"
1450:               end
1451:             elsif multiple && (is_ds || r.empty?)
1452:               # Not a query designed for this support, could be a valid regular query
1453:               super
1454:             else
1455:               raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}"
1456:             end
1457:           else
1458:             super
1459:           end
1460:         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 one_to_many or many_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 1361
1361:         def eager(*associations)
1362:           opt = @opts[:eager]
1363:           opt = opt ? opt.dup : {}
1364:           associations.flatten.each do |association|
1365:             case association
1366:               when Symbol
1367:                 check_association(model, association)
1368:                 opt[association] = nil
1369:               when Hash
1370:                 association.keys.each{|assoc| check_association(model, assoc)}
1371:                 opt.merge!(association)
1372:               else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
1373:             end
1374:           end
1375:           clone(:eager=>opt)
1376:         end

The secondary eager loading method. Loads all associations in a single query. This method should only be used if you need to filter or order 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 one_to_many or many_to_many associations, as you can create large cartesian products. If you must graph multiple one_to_many and many_to_many associations, make sure your filters are narrow 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 and one_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 table alias symbol keys and model object values).

[Source]

      # File lib/sequel/model/associations.rb, line 1399
1399:         def eager_graph(*associations)
1400:           ds = if @opts[:eager_graph]
1401:             self
1402:           else
1403:             # Each of the following have a symbol key for the table alias, with the following values: 
1404:             # :reciprocals - the reciprocal instance variable to use for this association
1405:             # :requirements - array of requirements for this association
1406:             # :alias_association_type_map - the type of association for this association
1407:             # :alias_association_name_map - the name of the association for this association
1408:             clone(:eager_graph=>{:requirements=>{}, :master=>alias_symbol(first_source), :alias_association_type_map=>{}, :alias_association_name_map=>{}, :reciprocals=>{}, :cartesian_product_number=>0})
1409:           end
1410:           ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations)
1411:         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.

[Source]

      # File lib/sequel/model/associations.rb, line 1466
1466:         def ungraphed
1467:           super.clone(:eager_graph=>nil)
1468:         end

Protected Instance methods

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 1484
1484:         def eager_graph_association(ds, model, ta, requirements, r, *associations)
1485:           assoc_name = r[:name]
1486:           assoc_table_alias = ds.unused_table_alias(assoc_name)
1487:           loader = r[:eager_grapher]
1488:           if !associations.empty?
1489:             if associations.first.respond_to?(:call)
1490:               callback = associations.first
1491:               associations = {}
1492:             elsif associations.length == 1 && (assocs = associations.first).is_a?(Hash) && assocs.length == 1 && (pr_assoc = assocs.to_a.first) && pr_assoc.first.respond_to?(:call)
1493:               callback, assoc = pr_assoc
1494:               associations = assoc.is_a?(Array) ? assoc : [assoc]
1495:             end
1496:           end
1497:           ds = if loader.arity == 1
1498:             loader.call(:self=>ds, :table_alias=>assoc_table_alias, :implicit_qualifier=>ta, :callback=>callback)
1499:           else
1500:             loader.call(ds, assoc_table_alias, ta)
1501:           end
1502:           ds = ds.order_more(*qualified_expression(r[:order], assoc_table_alias)) if r[:order] and r[:order_eager_graph]
1503:           eager_graph = ds.opts[:eager_graph]
1504:           eager_graph[:requirements][assoc_table_alias] = requirements.dup
1505:           eager_graph[:alias_association_name_map][assoc_table_alias] = assoc_name
1506:           eager_graph[:alias_association_type_map][assoc_table_alias] = r.returns_array?
1507:           eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2
1508:           ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty?
1509:           ds
1510:         end

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 1521
1521:         def eager_graph_associations(ds, model, ta, requirements, *associations)
1522:           return ds if associations.empty?
1523:           associations.flatten.each do |association|
1524:             ds = case association
1525:             when Symbol
1526:               ds.eager_graph_association(ds, model, ta, requirements, check_association(model, association))
1527:             when Hash
1528:               association.each do |assoc, assoc_assocs|
1529:                 ds = ds.eager_graph_association(ds, model, ta, requirements, check_association(model, assoc), assoc_assocs)
1530:               end
1531:               ds
1532:             else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
1533:             end
1534:           end
1535:           ds
1536:         end

Build associations out of the array of returned object graphs.

[Source]

      # File lib/sequel/model/associations.rb, line 1539
1539:         def eager_graph_build_associations(record_graphs)
1540:           eager_graph = @opts[:eager_graph]
1541:           master = eager_graph[:master]
1542:           requirements = eager_graph[:requirements]
1543:           alias_map = eager_graph[:alias_association_name_map]
1544:           type_map = eager_graph[:alias_association_type_map]
1545:           reciprocal_map = eager_graph[:reciprocals]
1546:       
1547:           # Make dependency map hash out of requirements array for each association.
1548:           # This builds a tree of dependencies that will be used for recursion
1549:           # to ensure that all parts of the object graph are loaded into the
1550:           # appropriate subordinate association.
1551:           dependency_map = {}
1552:           # Sort the associations by requirements length, so that
1553:           # requirements are added to the dependency hash before their
1554:           # dependencies.
1555:           requirements.sort_by{|a| a[1].length}.each do |ta, deps|
1556:             if deps.empty?
1557:               dependency_map[ta] = {}
1558:             else
1559:               deps = deps.dup
1560:               hash = dependency_map[deps.shift]
1561:               deps.each do |dep|
1562:                 hash = hash[dep]
1563:               end
1564:               hash[ta] = {}
1565:             end
1566:           end
1567:       
1568:           # This mapping is used to make sure that duplicate entries in the
1569:           # result set are mapped to a single record.  For example, using a
1570:           # single one_to_many association with 10 associated records,
1571:           # the main object will appear in the object graph 10 times.
1572:           # We map by primary key, if available, or by the object's entire values,
1573:           # if not. The mapping must be per table, so create sub maps for each table
1574:           # alias.
1575:           records_map = {master=>{}}
1576:           alias_map.keys.each{|ta| records_map[ta] = {}}
1577:       
1578:           # This will hold the final record set that we will be replacing the object graph with.
1579:           records = []
1580:           record_graphs.each do |record_graph|
1581:             primary_record = record_graph[master]
1582:             key = primary_record.pk_or_nil || primary_record.values.sort_by{|x| x[0].to_s}
1583:             if cached_pr = records_map[master][key]
1584:               primary_record = cached_pr
1585:             else
1586:               records_map[master][key] = primary_record
1587:               # Only add it to the list of records to return if it is a new record
1588:               records.push(primary_record)
1589:             end
1590:             # Build all associations for the current object and it's dependencies
1591:             eager_graph_build_associations_graph(dependency_map, alias_map, type_map, reciprocal_map, records_map, primary_record, record_graph)
1592:           end
1593:       
1594:           # Remove duplicate records from all associations if this graph could possibly be a cartesian product
1595:           eager_graph_make_associations_unique(records, dependency_map, alias_map, type_map) if eager_graph[:cartesian_product_number] > 1
1596:           
1597:           # Replace the array of object graphs with an array of model objects
1598:           record_graphs.replace(records)
1599:         end

[Validate]