This reverts commit 761bc751.

This commit wasn't fixing any issue just using the same table for
different models with different primary keys.

It has been moved to active_record_deprecated_finders.

Use #to_a instead.

Fixes #2832

This caused that `find_each` was producing extra db call taking all the records from db, and was less efficient than `ActiveRecord::Base#all`.

Currently if the code that calls .find_in_batches modifies the yielded array in place then .find_in_batches can enter an infinite loop searching with ruby object ids in the database instead of the primary key of records in the database. This happens because it naively assumes the yielded array hasn't been modified before calling #id on the last object in the array. And ruby (1.8 at least) alias' #id to #object_id so an integer is still returned no matter what the last object is.

By moving finding the #id of the last object before yielding the array it means the calling code can do whatever it wants to the array in terms of modifying it in place, and .find_in_batches doesn't care.

ActiveRecord::Relation#primary_key should return a string, just like ActiveRecord::Base.primary_key does.

corrected AR find_each and find_in_batches to raise when the user uses select but does not specify the primary key
Signed-off-by: NJosé Valim <jose.valim@gmail.com>