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

### Methods

eager eager_graph eager_graph_association eager_graph_associations eager_graph_build_associations extended ungraphed

### Public Class methods

extended(obj)

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

[Source]

          # File lib/sequel/model/associations.rb, line 1262
    1262:         def self.extended(obj)
    1263:           obj.def_mutation_method(:eager, :eager_graph)
    1264:         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 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 1292
    1292:         def eager(*associations)
    1293:           opt = @opts[:eager]
    1294:           opt = opt ? opt.dup : {}
    1295:           associations.flatten.each do |association|
    1296:             case association
    1297:               when Symbol
    1298:                 check_association(model, association)
    1299:                 opt[association] = nil
    1300:               when Hash
    1301:                 association.keys.each{|assoc| check_association(model, assoc)}
    1302:                 opt.merge!(association)
    1303:               else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    1304:             end
    1305:           end
    1306:           clone(:eager=>opt)
    1307:         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 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 1330
    1330:         def eager_graph(*associations)
    1331:           ds = if @opts[:eager_graph]
    1332:             self
    1333:           else
    1334:             # Each of the following have a symbol key for the table alias, with the following values:
    1335:             # :reciprocals - the reciprocal instance variable to use for this association
    1336:             # :requirements - array of requirements for this association
    1337:             # :alias_association_type_map - the type of association for this association
    1338:             # :alias_association_name_map - the name of the association for this association
    1339:             clone(:eager_graph=>{:requirements=>{}, :master=>alias_symbol(first_source), :alias_association_type_map=>{}, :alias_association_name_map=>{}, :reciprocals=>{}, :cartesian_product_number=>0})
    1340:           end
    1341:           ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations)
    1342:         end

ungraphed()

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 1348
    1348:         def ungraphed
    1349:           super.clone(:eager_graph=>nil)
    1350:         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 1366
    1366:         def eager_graph_association(ds, model, ta, requirements, r, *associations)
    1367:           klass = r.associated_class
    1368:           assoc_name = r[:name]
    1369:           assoc_table_alias = ds.unused_table_alias(assoc_name)
    1370:           ds = r[:eager_grapher].call(ds, assoc_table_alias, ta)
    1371:           ds = ds.order_more(*qualified_expression(r[:order], assoc_table_alias)) if r[:order] and r[:order_eager_graph]
    1372:           eager_graph = ds.opts[:eager_graph]
    1373:           eager_graph[:requirements][assoc_table_alias] = requirements.dup
    1374:           eager_graph[:alias_association_name_map][assoc_table_alias] = assoc_name
    1375:           eager_graph[:alias_association_type_map][assoc_table_alias] = r.returns_array?
    1376:           eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2
    1377:           ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty?
    1378:           ds
    1379:         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 1390
    1390:         def eager_graph_associations(ds, model, ta, requirements, *associations)
    1391:           return ds if associations.empty?
    1392:           associations.flatten.each do |association|
    1393:             ds = case association
    1394:             when Symbol
    1395:               ds.eager_graph_association(ds, model, ta, requirements, check_association(model, association))
    1396:             when Hash
    1397:               association.each do |assoc, assoc_assocs|
    1398:                 ds = ds.eager_graph_association(ds, model, ta, requirements, check_association(model, assoc), assoc_assocs)
    1399:               end
    1400:               ds
    1401:             else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    1402:             end
    1403:           end
    1404:           ds
    1405:         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 1408
    1408:         def eager_graph_build_associations(record_graphs)
    1409:           eager_graph = @opts[:eager_graph]
    1410:           master = eager_graph[:master]
    1411:           requirements = eager_graph[:requirements]
    1412:           alias_map = eager_graph[:alias_association_name_map]
    1413:           type_map = eager_graph[:alias_association_type_map]
    1414:           reciprocal_map = eager_graph[:reciprocals]
    1415:
    1416:           # Make dependency map hash out of requirements array for each association.
    1417:           # This builds a tree of dependencies that will be used for recursion
    1418:           # to ensure that all parts of the object graph are loaded into the
    1419:           # appropriate subordinate association.
    1420:           dependency_map = {}
    1421:           # Sort the associations by requirements length, so that
    1422:           # requirements are added to the dependency hash before their
    1423:           # dependencies.
    1424:           requirements.sort_by{|a| a[1].length}.each do |ta, deps|
    1425:             if deps.empty?
    1426:               dependency_map[ta] = {}
    1427:             else
    1428:               deps = deps.dup
    1429:               hash = dependency_map[deps.shift]
    1430:               deps.each do |dep|
    1431:                 hash = hash[dep]
    1432:               end
    1433:               hash[ta] = {}
    1434:             end
    1435:           end
    1436:
    1437:           # This mapping is used to make sure that duplicate entries in the
    1438:           # result set are mapped to a single record.  For example, using a
    1439:           # single one_to_many association with 10 associated records,
    1440:           # the main object will appear in the object graph 10 times.
    1441:           # We map by primary key, if available, or by the object's entire values,
    1442:           # if not. The mapping must be per table, so create sub maps for each table
    1443:           # alias.
    1444:           records_map = {master=>{}}
    1445:           alias_map.keys.each{|ta| records_map[ta] = {}}
    1446:
    1447:           # This will hold the final record set that we will be replacing the object graph with.
    1448:           records = []
    1449:           record_graphs.each do |record_graph|
    1450:             primary_record = record_graph[master]
    1451:             key = primary_record.pk_or_nil || primary_record.values.sort_by{|x| x[0].to_s}
    1452:             if cached_pr = records_map[master][key]
    1453:               primary_record = cached_pr
    1454:             else
    1455:               records_map[master][key] = primary_record
    1456:               # Only add it to the list of records to return if it is a new record
    1457:               records.push(primary_record)
    1458:             end
    1459:             # Build all associations for the current object and it's dependencies
    1460:             eager_graph_build_associations_graph(dependency_map, alias_map, type_map, reciprocal_map, records_map, primary_record, record_graph)
    1461:           end
    1462:
    1463:           # Remove duplicate records from all associations if this graph could possibly be a cartesian product
    1464:           eager_graph_make_associations_unique(records, dependency_map, alias_map, type_map) if eager_graph[:cartesian_product_number] > 1
    1465:
    1466:           # Replace the array of object graphs with an array of model objects
    1467:           record_graphs.replace(records)
    1468:         end

[Validate]