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` is usually 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

complex_expression_sql_append 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 1673
    1673:         def self.extended(obj)
    1674:           obj.def_mutation_method(:eager, :eager_graph)
    1675:         end

### Public Instance methods

complex_expression_sql_append(sql, op, args)

If the expression is in the form `x = y` where `y` is a `Sequel::Model` instance, array of `Sequel::Model` instances, or a `Sequel::Model` dataset, 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 1684
    1684:         def complex_expression_sql_append(sql, op, args)
    1685:           r = args.at(1)
    1686:           if (((op == '=''=' || op == '!=''!=') and r.is_a?(Sequel::Model)) ||
    1687:               (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)}))))
    1688:             l = args.at(0)
    1689:             if ar = model.association_reflections[l]
    1690:               if multiple
    1691:                 klass = ar.associated_class
    1692:                 if is_ds
    1693:                   if r.respond_to?(:model)
    1694:                     unless r.model <= klass
    1695:                       # A dataset for a different model class, could be a valid regular query
    1696:                       return super
    1697:                     end
    1698:                   else
    1699:                     # Not a model dataset, could be a valid regular query
    1700:                     return super
    1701:                   end
    1702:                 else
    1703:                   unless r.all?{|x| x.is_a?(klass)}
    1704:                     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}"
    1705:                   end
    1706:                 end
    1707:               elsif !r.is_a?(ar.associated_class)
    1708:                 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}"
    1709:               end
    1710:
    1711:               if exp = association_filter_expression(op, ar, r)
    1712:                 literal_append(sql, exp)
    1713:               else
    1714:                 raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}"
    1715:               end
    1716:             elsif multiple && (is_ds || r.empty?)
    1717:               # Not a query designed for this support, could be a valid regular query
    1718:               super
    1719:             else
    1720:               raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}"
    1721:             end
    1722:           else
    1723:             super
    1724:           end
    1725:         end

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 1753
    1753:         def eager(*associations)
    1754:           opt = @opts[:eager]
    1755:           opt = opt ? opt.dup : {}
    1756:           associations.flatten.each do |association|
    1757:             case association
    1758:               when Symbol
    1759:                 check_association(model, association)
    1760:                 opt[association] = nil
    1761:               when Hash
    1762:                 association.keys.each{|assoc| check_association(model, assoc)}
    1763:                 opt.merge!(association)
    1764:               else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    1765:             end
    1766:           end
    1767:           clone(:eager=>opt)
    1768:         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 uses `Dataset#graph` to create appropriate aliases for columns in all the tables. Then it uses the graph‘s metadata to build the associations from the single hash, and finally replaces the array of hashes with an array model objects inside all.

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`, it will yield plain hashes, each containing all columns from all the tables.

[Source]

          # File lib/sequel/model/associations.rb, line 1790
    1790:         def eager_graph(*associations)
    1791:           ds = if eg = @opts[:eager_graph]
    1792:             eg = eg.dup
    1793:             [:requirements, :reflections, :reciprocals].each{|k| eg[k] = eg[k].dup}
    1794:             clone(:eager_graph=>eg)
    1795:           else
    1796:             # Each of the following have a symbol key for the table alias, with the following values:
    1797:             # :reciprocals - the reciprocal instance variable to use for this association
    1798:             # :reflections - AssociationReflection instance related to this association
    1799:             # :requirements - array of requirements for this association
    1800:             clone(:eager_graph=>{:requirements=>{}, :master=>alias_symbol(first_source), :reflections=>{}, :reciprocals=>{}, :cartesian_product_number=>0})
    1801:           end
    1802:           ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations)
    1803:         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 1809
    1809:         def ungraphed
    1810:           super.clone(:eager_graph=>nil)
    1811:         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 1827
    1827:         def eager_graph_association(ds, model, ta, requirements, r, *associations)
    1828:           assoc_name = r[:name]
    1829:           assoc_table_alias = ds.unused_table_alias(r[:graph_alias_base])
    1830:           loader = r[:eager_grapher]
    1831:           if !associations.empty?
    1832:             if associations.first.respond_to?(:call)
    1833:               callback = associations.first
    1834:               associations = {}
    1835:             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)
    1836:               callback, assoc = pr_assoc
    1837:               associations = assoc.is_a?(Array) ? assoc : [assoc]
    1838:             end
    1839:           end
    1840:           ds = if loader.arity == 1
    1841:             loader.call(:self=>ds, :table_alias=>assoc_table_alias, :implicit_qualifier=>ta, :callback=>callback)
    1842:           else
    1843:             loader.call(ds, assoc_table_alias, ta)
    1844:           end
    1845:           ds = ds.order_more(*qualified_expression(r[:order], assoc_table_alias)) if r[:order] and r[:order_eager_graph]
    1846:           eager_graph = ds.opts[:eager_graph]
    1847:           eager_graph[:requirements][assoc_table_alias] = requirements.dup
    1848:           eager_graph[:reflections][assoc_table_alias] = r
    1849:           eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2
    1850:           ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty?
    1851:           ds
    1852:         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 1863
    1863:         def eager_graph_associations(ds, model, ta, requirements, *associations)
    1864:           return ds if associations.empty?
    1865:           associations.flatten.each do |association|
    1866:             ds = case association
    1867:             when Symbol
    1868:               ds.eager_graph_association(ds, model, ta, requirements, check_association(model, association))
    1869:             when Hash
    1870:               association.each do |assoc, assoc_assocs|
    1871:                 ds = ds.eager_graph_association(ds, model, ta, requirements, check_association(model, assoc), assoc_assocs)
    1872:               end
    1873:               ds
    1874:             else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    1875:             end
    1876:           end
    1877:           ds
    1878:         end

eager_graph_build_associations(hashes)

Replace the array of plain hashes with an array of model objects will all eager_graphed associations set in the associations cache for each object.

[Source]

          # File lib/sequel/model/associations.rb, line 1882
    1882:         def eager_graph_build_associations(hashes)
    1883:           hashes.replace(EagerGraphLoader.new(self).load(hashes))
    1884:         end

[Validate]