We shouldn't be using scoped.scoping { ... } to build associated records, as this can affect validations/callbacks/etc inside the record itself [#6252 state:resolved]

Construct an actual ActiveRecord::Relation object for the association scope, rather than a hash which is passed to apply_finder_options. This allows more flexibility in how the scope is created, for example because scope.where(a, b) and scope.where(a).where(b) mean different things.

Let AssociationCollection#find use #scoped to do its finding. Note that I am removing test_polymorphic_has_many_going_through_join_model_with_disabled_include, since this specifies different behaviour for an association than for a regular scope. It seems reasonable to expect scopes and association proxies to behave in roughly the same way rather than having subtle differences.

Specify the STI type condition using SQL IN rather than a whole load of ORs. Required a fix to ActiveRecord::Relation#merge for properly merging create_with_value. This also fixes a situation where the type condition was appearing twice in the resultant SQL query.

Associations: DRY up the code which is generating conditions, and make it all use arel rather than SQL strings

When a has_many association is not :uniq, appending the same record multiple times should append it to the @target multiple times [#5964 state:resolved]

Delete create, create! and create_record from HasManyThroughAssociation in exchange for more generic versions in AssociationCollection

Specify insert_record with NotImplementedError in AssociationCollection, to indicate that subclasses should implement it. Also add save_record to reduce duplication.

* rename method names (actually in tests)
* rename instance variable @_named_scopes_cache to @_scopes_cache
* rename references in doc comments
* don't touch CHANGELOG :)