diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 9e22be1b947920181696b87c5bbd11d7e42f3c2f..c6a6e44724d9eba627643bd75fce6b0d00d19c1b 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,31 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Relation#merge now only overwrites where values on the LHS of the
+    merge. Consider:
+
+        left  = Person.where(age: [13, 14, 15])
+        right = Person.where(age: [13, 14]).where(age: [14, 15])
+
+    `left` results in the following SQL:
+
+        WHERE age IN (13, 14, 15)
+
+    `right` results in the following SQL:
+
+        WHERE age IN (13, 14) AND age IN (14, 15)
+
+    Previously, `left.merge(right)` would result in all but the last
+    condition being removed:
+
+        WHERE age IN (14, 15)
+
+    Now it results in the LHS condition(s) for `age` being removed, but
+    the RHS remains as it is:
+
+        WHERE age IN (13, 14) AND age IN (14, 15)
+
+    *Jon Leighton*
+
 *   Fix handling of dirty time zone aware attributes
 
     Previously, when `time_zone_aware_attributes` were enabled, after
diff --git a/activerecord/lib/active_record/associations/association_scope.rb b/activerecord/lib/active_record/associations/association_scope.rb
index 300f67959d646e4396d4167656831b7dfa0954d3..c5fb1fe2c7330ae4c5d61be94dddfb2d8c6bface 100644
--- a/activerecord/lib/active_record/associations/association_scope.rb
+++ b/activerecord/lib/active_record/associations/association_scope.rb
@@ -15,7 +15,6 @@ def initialize(association)
 
       def scope
         scope = klass.unscoped
-        scope.merge! eval_scope(klass, reflection.scope) if reflection.scope
         scope.extending! Array(options[:extend])
         add_constraints(scope)
       end
@@ -59,7 +58,7 @@ def add_constraints(scope)
 
           if reflection.source_macro == :belongs_to
             if reflection.options[:polymorphic]
-              key = reflection.association_primary_key(klass)
+              key = reflection.association_primary_key(self.klass)
             else
               key = reflection.association_primary_key
             end
@@ -92,8 +91,13 @@ def add_constraints(scope)
 
           # Exclude the scope of the association itself, because that
           # was already merged in the #scope method.
-          (scope_chain[i] - [self.reflection.scope]).each do |scope_chain_item|
-            item = eval_scope(reflection.klass, scope_chain_item)
+          scope_chain[i].each do |scope_chain_item|
+            klass = i == 0 ? self.klass : reflection.klass
+            item  = eval_scope(klass, scope_chain_item)
+
+            if scope_chain_item == self.reflection.scope
+              scope.merge! item.except(:where, :includes)
+            end
 
             scope.includes! item.includes_values
             scope.where_values += item.where_values
diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 59226d316ebc2708d38415652b155afbccf604b5..eb23e92fb8b2caf0421a31fe70cb052d3d1735f1 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -1,4 +1,5 @@
 require 'active_support/core_ext/hash/keys'
+require "set"
 
 module ActiveRecord
   class Relation
@@ -105,25 +106,26 @@ def merged_binds
       end
 
       def merged_wheres
-        if values[:where]
-          merged_wheres = relation.where_values + values[:where]
-
-          unless relation.where_values.empty?
-            # Remove equalities with duplicated left-hand. Last one wins.
-            seen = {}
-            merged_wheres = merged_wheres.reverse.reject { |w|
-              nuke = false
-              if w.respond_to?(:operator) && w.operator == :==
-                nuke         = seen[w.left]
-                seen[w.left] = true
-              end
-              nuke
-            }.reverse
-          end
+        values[:where] ||= []
 
-          merged_wheres
+        if values[:where].empty? || relation.where_values.empty?
+          relation.where_values + values[:where]
         else
-          relation.where_values
+          # Remove equalities from the existing relation with a LHS which is
+          # present in the relation being merged in.
+
+          seen = Set.new
+          values[:where].each { |w|
+            if w.respond_to?(:operator) && w.operator == :==
+              seen << w.left
+            end
+          }
+
+          relation.where_values.reject { |w|
+            w.respond_to?(:operator) &&
+              w.operator == :== &&
+              seen.include?(w.left)
+          } + values[:where]
         end
       end
     end
diff --git a/activerecord/test/cases/associations/has_many_through_associations_test.rb b/activerecord/test/cases/associations/has_many_through_associations_test.rb
index af91fb29208f7dded5b0e03dc72a0d8c9a7713bf..67d18f313a0ce1204a2f68b81c485f57887ed992 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -894,4 +894,11 @@ def test_has_many_through_with_polymorphic_source
     end
   end
 
+  test "has many through with default scope on the target" do
+    person = people(:michael)
+    assert_equal [posts(:thinking)], person.first_posts
+
+    readers(:michael_authorless).update(first_post_id: 1)
+    assert_equal [posts(:thinking)], person.reload.first_posts
+  end
 end
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 64f1c9f6ccf8832433721da3e5024ac842e4d374..379c0c0758cf42b06d463f5bd0122436e1ba85de 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -1488,4 +1488,17 @@ def __omg__
       Array.send(:remove_method, :__omg__)
     end
   end
+
+  test "merge collapses wheres from the LHS only" do
+    left  = Post.where(title: "omg").where(comments_count: 1)
+    right = Post.where(title: "wtf").where(title: "bbq")
+
+    expected = [left.where_values[1]] + right.where_values
+    merged   = left.merge(right)
+
+    assert_equal expected, merged.where_values
+    assert !merged.to_sql.include?("omg")
+    assert merged.to_sql.include?("wtf")
+    assert merged.to_sql.include?("bbq")
+  end
 end
diff --git a/activerecord/test/fixtures/readers.yml b/activerecord/test/fixtures/readers.yml
index 8a6076655b7b23a789ae8b2c698b6d56eb3c9db2..14b883f04104649cf4462467ee34a340d82f0a11 100644
--- a/activerecord/test/fixtures/readers.yml
+++ b/activerecord/test/fixtures/readers.yml
@@ -2,8 +2,10 @@ michael_welcome:
   id: 1
   post_id: 1
   person_id: 1
+  first_post_id: 2
 
 michael_authorless:
   id: 2
   post_id: 3
-  person_id: 1
\ No newline at end of file
+  person_id: 1
+  first_post_id: 3
diff --git a/activerecord/test/models/person.rb b/activerecord/test/models/person.rb
index c602ca5eac9662ae4c1e66d7d204177518cc20bc..fa717ef8d6944bc39da437909e6e6a1acf3c1429 100644
--- a/activerecord/test/models/person.rb
+++ b/activerecord/test/models/person.rb
@@ -15,6 +15,7 @@ class Person < ActiveRecord::Base
   has_many :fixed_bad_references, -> { where :favourite => true }, :class_name => 'BadReference'
   has_one  :favourite_reference, -> { where 'favourite=?', true }, :class_name => 'Reference'
   has_many :posts_with_comments_sorted_by_comment_id, -> { includes(:comments).order('comments.id') }, :through => :readers, :source => :post
+  has_many :first_posts, -> { where(id: [1, 2]) }, through: :readers
 
   has_many :jobs, :through => :references
   has_many :jobs_with_dependent_destroy,    :source => :job, :through => :references, :dependent => :destroy
diff --git a/activerecord/test/models/reader.rb b/activerecord/test/models/reader.rb
index f8fb9c573e0d2eae9a542493a5d33ce55f06b9b7..3a6b7fad343802ae51beda1f163e36c5685c5ef2 100644
--- a/activerecord/test/models/reader.rb
+++ b/activerecord/test/models/reader.rb
@@ -2,6 +2,7 @@ class Reader < ActiveRecord::Base
   belongs_to :post
   belongs_to :person, :inverse_of => :readers
   belongs_to :single_person, :class_name => 'Person', :foreign_key => :person_id, :inverse_of => :reader
+  belongs_to :first_post, -> { where(id: [2, 3]) }
 end
 
 class SecureReader < ActiveRecord::Base
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 46219c53dbd27a8f6e1767b4e6fe5ae97cbb9f09..cd9835259a08592177de8f534bd7a6afc18884d2 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -567,6 +567,7 @@ def create_table(*args, &block)
     t.integer :post_id, :null => false
     t.integer :person_id, :null => false
     t.boolean :skimmer, :default => false
+    t.integer :first_post_id
   end
 
   create_table :references, :force => true do |t|