ActiveRecord scope merge behavior

3.2.12 - 3.2.16

Scopes that are
  -- combined via #merge AND
  -- are simple column/value equality tests
will be MERGED, last equality test wins.

Chained scopes don't always use merge:
  -- explicit where clauses don't use merge  (=> AND)
  -- class methods (vs scopes) don't use merge ( => AND)

Chained scopes sometimes use merge:
  -- scope references (vs class methods) use MERGE logic
  -- an explicit merge, uses MERGE logic

An explicit merge will cause ALL chained relations to be merged (class, explicit, scope)
	Model.a.merge(Model.c.d)    #  a.merge(c).merge(d)

4.0.0 and beyond

equality scopes are ONLY merged via an explicit use of merge