1 files changed, 167 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
new file mode 100644
index 0000000000..c0639742be
--- /dev/null
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -0,0 +1,167 @@
+module ActiveRecord
+  module Associations
+    class Preloader
+      class Association #:nodoc:
+        attr_reader :owners, :reflection, :preload_scope, :model, :klass
+        attr_reader :preloaded_records
+
+        def initialize(klass, owners, reflection, preload_scope)
+          @klass         = klass
+          @owners        = owners
+          @reflection    = reflection
+          @preload_scope = preload_scope
+          @model         = owners.first && owners.first.class
+          @scope         = nil
+          @owners_by_key = nil
+          @preloaded_records = []
+        end
+
+        def run(preloader)
+          preload(preloader)
+        end
+
+        def preload(preloader)
+          raise NotImplementedError
+        end
+
+        def scope
+          @scope ||= build_scope
+        end
+
+        def records_for(ids)
+          query_scope(ids)
+        end
+
+        def query_scope(ids)
+          scope.where(association_key.in(ids))
+        end
+
+        def table
+          klass.arel_table
+        end
+
+        # The name of the key on the associated records
+        def association_key_name
+          raise NotImplementedError
+        end
+
+        # This is overridden by HABTM as the condition should be on the foreign_key column in
+        # the join table
+        def association_key
+          table[association_key_name]
+        end
+
+        # The name of the key on the model which declares the association
+        def owner_key_name
+          raise NotImplementedError
+        end
+
+        def owners_by_key
+          @owners_by_key ||= if key_conversion_required?
+                               owners.group_by do |owner|
+                                 owner[owner_key_name].to_s
+                               end
+                             else
+                               owners.group_by do |owner|
+                                 owner[owner_key_name]
+                               end
+                             end
+        end
+
+        def options
+          reflection.options
+        end
+
+        private
+
+        def associated_records_by_owner(preloader)
+          owners_map = owners_by_key
+          owner_keys = owners_map.keys.compact
+
+          # Each record may have multiple owners, and vice-versa
+          records_by_owner = owners.each_with_object({}) do |owner,h|
+            h[owner] = []
+          end
+
+          if owner_keys.any?
+            # Some databases impose a limit on the number of ids in a list (in Oracle it's 1000)
+            # Make several smaller queries if necessary or make one query if the adapter supports it
+            sliced  = owner_keys.each_slice(klass.connection.in_clause_length || owner_keys.size)
+
+            records = load_slices sliced
+            records.each do |record, owner_key|
+              owners_map[owner_key].each do |owner|
+                records_by_owner[owner] << record
+              end
+            end
+          end
+
+          records_by_owner
+        end
+
+        def key_conversion_required?
+          association_key_type != owner_key_type
+        end
+
+        def association_key_type
+          @klass.type_for_attribute(association_key_name.to_s).type
+        end
+
+        def owner_key_type
+          @model.type_for_attribute(owner_key_name.to_s).type
+        end
+
+        def load_slices(slices)
+          @preloaded_records = slices.flat_map { |slice|
+            records_for(slice)
+          }
+
+          @preloaded_records.map { |record|
+            key = record[association_key_name]
+            key = key.to_s if key_conversion_required?
+
+            [record, key]
+          }
+        end
+
+        def reflection_scope
+          @reflection_scope ||= reflection.scope ? klass.unscoped.instance_exec(nil, &reflection.scope) : klass.unscoped
+        end
+
+        def build_scope
+          scope = klass.unscoped
+
+          values         = reflection_scope.values
+          reflection_binds = reflection_scope.bind_values
+          preload_values = preload_scope.values
+          preload_binds  = preload_scope.bind_values
+
+          scope.where_values      = Array(values[:where])      + Array(preload_values[:where])
+          scope.references_values = Array(values[:references]) + Array(preload_values[:references])
+          scope.bind_values       = (reflection_binds + preload_binds)
+
+          scope._select!   preload_values[:select] || values[:select] || table[Arel.star]
+          scope.includes! preload_values[:includes] || values[:includes]
+
+          if preload_values.key? :order
+            scope.order! preload_values[:order]
+          else
+            if values.key? :order
+              scope.order! values[:order]
+            end
+          end
+
+          if preload_values[:readonly] || values[:readonly]
+            scope.readonly!
+          end
+
+          if options[:as]
+            scope.where!(klass.table_name => { reflection.type => model.base_class.sti_name })
+          end
+
+          klass.default_scoped.merge(scope)
+        end
+      end
+    end
+  end
+end