rails/rails — pull request #17012 by petedmarsh (2014-09-22 19:19:03+00:00)
Title: Clarify the documentation of the primary_key option for associations

Previously the documentation stated that `primary_key` should be the name of
a *method* that returns the primary key used for the association. This is
incorrect. This changes the documentation to say that the value must be the
name of a column.

This fixes: https://github.com/rails/rails/issues/17004.