Fix the `:primary_key` option for `has_many` associations.

When removing records from a `has_many` association it used
the `primary_key` defined on the association.

Our test suite didn't fail because on all occurences of `:primary_key`,
the specified column was available in both tables. This prevented the
code from raising an exception but it still behaved badly.

I added a test-case to prevent regressions that failed with:

```
  1) Error:
HasManyAssociationsTest#test_has_many_assignment_with_custom_primary_key:
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: essays.first_name: UPDATE "essays" SET "writer_id" = NULL WHERE "essays"."writer_id" = ? AND "essays"."first_name" IS NULL
```

2 files changed, 9 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index d85570236f..058c812dca 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -1472,6 +1472,14 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_equal david.essays, Essay.where(writer_id: "David")
   end
 
+  def test_has_many_assignment_with_custom_primary_key
+    david = people(:david)
+
+    assert_equal ["A Modest Proposal"], david.essays.map(&:name)
+    david.essays = [Essay.create!(name: "Remote Work" )]
+    assert_equal ["Remote Work"], david.essays.map(&:name)
+  end
+
   def test_blank_custom_primary_key_on_new_record_should_not_run_queries
     author = Author.new
     assert !author.essays.loaded?
diff --git a/activerecord/test/models/person.rb b/activerecord/test/models/person.rb
index 2985160d28..1a282dbce4 100644
--- a/activerecord/test/models/person.rb
+++ b/activerecord/test/models/person.rb
@@ -32,6 +32,7 @@ class Person < ActiveRecord::Base
 
   has_many :agents_posts,         :through => :agents,       :source => :posts
   has_many :agents_posts_authors, :through => :agents_posts, :source => :author
+  has_many :essays, primary_key: "first_name", foreign_key: "writer_id"
 
   scope :males,   -> { where(:gender => 'M') }
   scope :females, -> { where(:gender => 'F') }