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

Set the create scope to an empty hash in ThroughAssociationScope. For reasoning please see the inline code comments.

Use conditionals and implicit returns rather than explicit returns and postfix ifs (it's easier to read)

Remove has_cached_counter? and cached_counter_attribute_name from HasManyThroughAssociation, as the exact same methods are inherited from HasManyAssociation

Use the through association proxy for operations on the through record, so that those operations are automatically scoped and therefore construct_join_attributes does not need to use construct_owner_attributes.

If a has_many goes :through a belongs_to, and the foreign key of the belongs_to changes, then the has_many should be considered stale.

Improved strategy for updating a belongs_to association when the foreign key changes. Rather than resetting each affected association when the foreign key changes, we should lazily check for 'staleness' (where fk does not match target id) when the association is accessed.

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]

Fix creation of has_many through records with custom primary_key option on belongs_to [#2990 state:resolved]

has_many associations with :dependent => :delete_all should update the counter cache when deleting records

Fix problem where wrong keys are used in JoinAssociation when an association goes :through a belongs_to [#2801 state:resolved]

Remove unnecessary overloaded methods create, create! and create_record from HasAndBelongsToManyAssociation

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 :)

class names are already strings, so we do not need to call to_s on the strings that are already strings