Better support for `where()` conditions that use an association name.

Using the name of an association in `where` previously worked only
if the value was a single `ActiveRecrd::Base` object. e.g.

    Post.where(author: Author.first)

Any other values, including `nil`, would cause invalid SQL to be
generated. This change supports arguments in the `where` query
conditions where the key is a `belongs_to` association name and the
value is `nil`, an `Array` of `ActiveRecord::Base` objects, or an
`ActiveRecord::Relation` object.

    # Given the Post model
    class Post < ActiveRecord::Base
      belongs_to :author
    end

    # nil value finds records where the association is not set
    Post.where(author: nil)
    # SELECT "posts".* FROM "posts" WHERE "posts"."author_id" IS NULL

    # Array values find records where the association foreign key
    # matches the ids of the passed ActiveRecord models, resulting
    # in the same query as Post.where(author_id: [1,2])
    authors_array = [Author.find(1), Author.find(2)]
    Post.where(author: authors_array)

    # ActiveRecord::Relation values find records using the same
    # query as Post.where(author_id: Author.where(last_name: "Emde"))
    Post.where(author: Author.where(last_name: "Emde"))

Polymorphic `belongs_to` associations will continue to be handled
appropriately, with the polymorphic `association_type` field added
to the query to match the base class of the value. This feature
previously only worked when the value was a single `ActveRecord::Base`.

    class Post < ActiveRecord::Base
      belongs_to :author, polymorphic: true
    end

    Post.where(author: Author.where(last_name: "Emde"))
    # Generates a query similar to:
    Post.where(author_id: Author.where(last_name: "Emde"), author_type: "Author")

1 files changed, 15 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/relation/predicate_builder.rb b/activerecord/lib/active_record/relation/predicate_builder.rb
index c60cd27a83..1252af7635 100644
--- a/activerecord/lib/active_record/relation/predicate_builder.rb
+++ b/activerecord/lib/active_record/relation/predicate_builder.rb
@@ -55,9 +55,9 @@ module ActiveRecord
       #
       # For polymorphic relationships, find the foreign key and type:
       # PriceEstimate.where(estimate_of: treasure)
-      if klass && value.is_a?(Base) && reflection = klass.reflect_on_association(column.to_sym)
-        if reflection.polymorphic?
-          queries << build(table[reflection.foreign_type], value.class.base_class)
+      if klass && reflection = klass.reflect_on_association(column.to_sym)
+        if reflection.polymorphic? && base_class = polymorphic_base_class_from_value(value)
+          queries << build(table[reflection.foreign_type], base_class)
         end
 
         column = reflection.foreign_key
@@ -67,6 +67,18 @@ module ActiveRecord
       queries
     end
 
+    def self.polymorphic_base_class_from_value(value)
+      case value
+      when Relation
+        value.klass.base_class
+      when Array
+        val = value.compact.first
+        val.class.base_class if val.is_a?(Base)
+      when Base
+        value.class.base_class
+      end
+    end
+
     def self.references(attributes)
       attributes.map do |key, value|
         if value.is_a?(Hash)