Closes #3104.

Ensure that the foreign key gets set when doing record.create_association or record.create_association. Fixes #1960.

Refactor Active Record test connection setup. Please see the RUNNING_UNIT_TESTS file for details, but essentially you can now configure things in test/config.yml. You can also run tests directly via the command line, e.g. ruby path/to/test.rb (no rake needed, uses default db connection from test/config.yml). This will help us fix the CI by enabling us to isolate the different Rails versions to different databases.

Split AssociationProxy into an Association class (and subclasses) which manages the association, and a CollectionProxy class which is *only* a proxy. Singular associations no longer have a proxy. See CHANGELOG for more.

This required changing the code to keep the association proxy for a belongs_to around, despite its target being nil. Which in turn required various changes to the way that stale target checking is handled, in order to support various edge cases (loaded target is nil then foreign key added, foreign key is changed and then changed back, etc). A side effect is that the code is nicer and more succinct.

Note that I am removing test_no_unexpected_aliasing since that is basically checking that the proxy for a belongs_to *does* change, which is the exact opposite of the intention of this commit. Also adding various tests for various edge cases and related things.

Phew, long commit message!

Some basic tests for the :foreign_type option on belongs_to, which was previously completely untested.

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.

This reverts commit 4db9dca55e3acc2c59f252eb83ecb83db5f4b81b.

Conflicts:

	activerecord/test/cases/identity_map_test.rb

- persisted? is the API defined in ActiveModel
- makes it easier for extension libraries to conform to ActiveModel APIs
  without concern for whether the extended object is specifically
  ActiveRecord

[#5927 state:committed]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

Convert :primary_key in association to a string before comparing to column names, so that for example :primary_key => :another_pk works as well [#5605 state:resolved]

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

code gardening: we have assert_(nil|blank|present), more concise, with better default failure messages - let's use them

[#2786 state:committed]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

[#5153 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

[#4645 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

[#765 state:committed]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>