Ambiguous reflections are on :through relationships are no longer supported.

For example, you need to change this:

  class Author < ActiveRecord::Base
    has_many :posts
    has_many :taggings, :through => :posts
  end

  class Post < ActiveRecord::Base
    has_one :tagging
    has_many :taggings
  end

  class Tagging < ActiveRecord::Base
  end

To this:

  class Author < ActiveRecord::Base
    has_many :posts
    has_many :taggings, :through => :posts, :source => :tagging
  end

  class Post < ActiveRecord::Base
    has_one :tagging
    has_many :taggings
  end

  class Tagging < ActiveRecord::Base
  end

Author: tenderlove

activerecord/CHANGELOG.md

@@ -1,3 +1,34 @@
+*   Ambiguous reflections are on :through relationships are no longer supported.
+    For example, you need to change this:
+
+      class Author < ActiveRecord::Base
+        has_many :posts
+        has_many :taggings, :through => :posts
+      end
+      
+      class Post < ActiveRecord::Base
+        has_one :tagging
+        has_many :taggings
+      end
+      
+      class Tagging < ActiveRecord::Base
+      end
+
+    To this:
+
+      class Author < ActiveRecord::Base
+        has_many :posts
+        has_many :taggings, :through => :posts, :source => :tagging
+      end
+      
+      class Post < ActiveRecord::Base
+        has_one :tagging
+        has_many :taggings
+      end
+      
+      class Tagging < ActiveRecord::Base
+      end
+
 *   Remove column restrictions for `count`, let the database raise if the SQL is
     invalid. The previous behavior was untested and surprising for the user.
     Fixes #5554.

activerecord/lib/active_record/reflection.rb

@@ -499,6 +499,11 @@ class ThroughReflection < AssociationReflection #:nodoc:
       delegate :foreign_key, :foreign_type, :association_foreign_key,
                :active_record_primary_key, :type, :to => :source_reflection
 
+      def initialize(macro, name, scope, options, active_record)
+        super
+        @source_reflection = nil
+      end
+
       # Returns the source of the through reflection. It checks both a singularized
       # and pluralized form for <tt>:belongs_to</tt> or <tt>:has_many</tt>.
       #
@@ -517,7 +522,28 @@ class ThroughReflection < AssociationReflection #:nodoc:
       #   # => <ActiveRecord::Reflection::AssociationReflection: @macro=:belongs_to, @name=:tag, @active_record=Tagging, @plural_name="tags">
       #
       def source_reflection
-        @source_reflection ||= source_reflection_names.collect { |name| through_reflection.klass.reflect_on_association(name) }.compact.first
+        return @source_reflection if @source_reflection
+
+        reflections = source_reflection_names.collect { |name|
+          through_reflection.klass.reflect_on_association(name)
+        }.compact
+
+        if reflections.length > 1
+          example_options = options.dup
+          example_options[:source] = source_reflection_names.first
+          ActiveSupport::Deprecation.warn <<-eowarn
+Ambiguous source reflection for through association.  Please specify a :source
+directive on your declaration like:
+
+  class #{active_record.name} < ActiveRecord::Base
+    #{macro} :#{name}, #{example_options}
+  end
+
+          eowarn
+          @source_reflection = reflections.first
+        else
+          @source_reflection = reflections.first
+        end
       end
 
       # Returns the AssociationReflection object specified in the <tt>:through</tt> option

activerecord/test/models/author.rb

@@ -85,7 +85,7 @@ class Author < ActiveRecord::Base
   has_many :author_favorites
   has_many :favorite_authors, -> { order('name') }, :through => :author_favorites
 
-  has_many :taggings,        :through => :posts
+  has_many :taggings,        :through => :posts, :source => :taggings
   has_many :taggings_2,      :through => :posts, :source => :tagging
   has_many :tags,            :through => :posts
   has_many :post_categories, :through => :posts, :source => :categories

activerecord/test/models/company.rb

@@ -141,7 +141,7 @@ class Client < Company
   belongs_to :firm_with_primary_key_symbols, :class_name => "Firm", :primary_key => :name, :foreign_key => :firm_name
   belongs_to :readonly_firm, -> { readonly }, :class_name => "Firm", :foreign_key => "firm_id"
   belongs_to :bob_firm, -> { where :name => "Bob" }, :class_name => "Firm", :foreign_key => "client_of"
-  has_many :accounts, :through => :firm
+  has_many :accounts, :through => :firm, :source => :accounts
   belongs_to :account
 
   class RaisedOnSave < RuntimeError; end