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 1747
    1747:         def self.extended(obj)
    1748:           obj.def_mutation_method(:eager, :eager_graph)
    1749:         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 1758
    1758:         def complex_expression_sql_append(sql, op, args)
    1759:           r = args.at(1)
    1760:           if (((op == '=''=' || op == '!=''!=') and r.is_a?(Sequel::Model)) ||
    1761:               (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)}))))
    1762:             l = args.at(0)
    1763:             if ar = model.association_reflections[l]
    1764:               if multiple
    1765:                 klass = ar.associated_class
    1766:                 if is_ds
    1767:                   if r.respond_to?(:model)
    1768:                     unless r.model <= klass
    1769:                       # A dataset for a different model class, could be a valid regular query
    1770:                       return super
    1771:                     end
    1772:                   else
    1773:                     # Not a model dataset, could be a valid regular query
    1774:                     return super
    1775:                   end
    1776:                 else
    1777:                   unless r.all?{|x| x.is_a?(klass)}
    1778:                     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}"
    1779:                   end
    1780:                 end
    1781:               elsif !r.is_a?(ar.associated_class)
    1782:                 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}"
    1783:               end
    1784:
    1785:               if exp = association_filter_expression(op, ar, r)
    1786:                 literal_append(sql, exp)
    1787:               else
    1788:                 raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}"
    1789:               end
    1790:             elsif multiple && (is_ds || r.empty?)
    1791:               # Not a query designed for this support, could be a valid regular query
    1792:               super
    1793:             else
    1794:               raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}"
    1795:             end
    1796:           else
    1797:             super
    1798:           end
    1799:         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. If the association uses a block or has an :eager_block argument, it is used.

[Source]

          # File lib/sequel/model/associations.rb, line 1826
    1826:         def eager(*associations)
    1827:           opt = @opts[:eager]
    1828:           opt = opt ? opt.dup : {}
    1829:           associations.flatten.each do |association|
    1830:             case association
    1831:               when Symbol
    1832:                 check_association(model, association)
    1833:                 opt[association] = nil
    1834:               when Hash
    1835:                 association.keys.each{|assoc| check_association(model, assoc)}
    1836:                 opt.merge!(association)
    1837:               else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    1838:             end
    1839:           end
    1840:           clone(:eager=>opt)
    1841:         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 1863
    1863:         def eager_graph(*associations)
    1864:           ds = if eg = @opts[:eager_graph]
    1865:             eg = eg.dup
    1866:             [:requirements, :reflections, :reciprocals].each{|k| eg[k] = eg[k].dup}
    1867:             clone(:eager_graph=>eg)
    1868:           else
    1869:             # Each of the following have a symbol key for the table alias, with the following values:
    1870:             # :reciprocals - the reciprocal instance variable to use for this association
    1871:             # :reflections - AssociationReflection instance related to this association
    1872:             # :requirements - array of requirements for this association
    1873:             clone(:eager_graph=>{:requirements=>{}, :master=>alias_symbol(first_source), :reflections=>{}, :reciprocals=>{}, :cartesian_product_number=>0})
    1874:           end
    1875:           ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations)
    1876:         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 1882
    1882:         def ungraphed
    1883:           super.clone(:eager_graph=>nil)
    1884:         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, or an SQL::AliasedExpression with the reflection as the expression and the alias base as the aliaz.
*associations : | any associations dependent on this one

[Source]

          # File lib/sequel/model/associations.rb, line 1901
    1901:         def eager_graph_association(ds, model, ta, requirements, r, *associations)
    1902:           if r.is_a?(SQL::AliasedExpression)
    1903:             alias_base = r.aliaz
    1904:             r = r.expression
    1905:           else
    1906:             alias_base = r[:graph_alias_base]
    1907:           end
    1908:           assoc_name = r[:name]
    1909:           assoc_table_alias = ds.unused_table_alias(alias_base)
    1910:           loader = r[:eager_grapher]
    1911:           if !associations.empty?
    1912:             if associations.first.respond_to?(:call)
    1913:               callback = associations.first
    1914:               associations = {}
    1915:             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)
    1916:               callback, assoc = pr_assoc
    1917:               associations = assoc.is_a?(Array) ? assoc : [assoc]
    1918:             end
    1919:           end
    1920:           ds = if loader.arity == 1
    1921:             loader.call(:self=>ds, :table_alias=>assoc_table_alias, :implicit_qualifier=>ta, :callback=>callback)
    1922:           else
    1923:             loader.call(ds, assoc_table_alias, ta)
    1924:           end
    1925:           ds = ds.order_more(*qualified_expression(r[:order], assoc_table_alias)) if r[:order] and r[:order_eager_graph]
    1926:           eager_graph = ds.opts[:eager_graph]
    1927:           eager_graph[:requirements][assoc_table_alias] = requirements.dup
    1928:           eager_graph[:reflections][assoc_table_alias] = r
    1929:           eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2
    1930:           ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty?
    1931:           ds
    1932:         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 1943
    1943:         def eager_graph_associations(ds, model, ta, requirements, *associations)
    1944:           return ds if associations.empty?
    1945:           associations.flatten.each do |association|
    1946:             ds = case association
    1947:             when Symbol, SQL::AliasedExpression
    1948:               ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, association))
    1949:             when Hash
    1950:               association.each do |assoc, assoc_assocs|
    1951:                 ds = ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, assoc), assoc_assocs)
    1952:               end
    1953:               ds
    1954:             else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    1955:             end
    1956:           end
    1957:           ds
    1958:         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 1962
    1962:         def eager_graph_build_associations(hashes)
    1963:           hashes.replace(EagerGraphLoader.new(self).load(hashes))
    1964:         end

[Validate]