Simplify/fix implementation of default scopes

The previous implementation was necessary in order to support stuff
like:

    class Post < ActiveRecord::Base
      default_scope where(published: true)
      scope :ordered, order("created_at")
    end

If we didn't evaluate the default scope at the last possible moment
before sending the SQL to the database, it would become impossible to
do:

    Post.unscoped.ordered

This is because the default scope would already be bound up in the
"ordered" scope, and therefore wouldn't be removed by the
"Post.unscoped" part.

In 4.0, we have deprecated all "eager" forms of scopes. So now you must
write:

    class Post < ActiveRecord::Base
      default_scope { where(published: true) }
      scope :ordered, -> { order("created_at") }
    end

This prevents the default scope getting bound up inside the "ordered"
scope, which means we can now have a simpler/better/more natural
implementation of default scoping.

A knock on effect is that some things that didn't work properly now do.
For example it was previously impossible to use #except to remove a part
of the default scope, since the default scope was evaluated after the
call to #except.

1 files changed, 15 insertions, 35 deletions

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index d37471e9ad..e6bfa2a969 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -17,17 +17,14 @@ module ActiveRecord
     include FinderMethods, Calculations, SpawnMethods, QueryMethods, Batches, Explain, Delegation
 
     attr_reader :table, :klass, :loaded
-    attr_accessor :default_scoped
     alias :model :klass
     alias :loaded? :loaded
-    alias :default_scoped? :default_scoped
 
     def initialize(klass, table, values = {})
-      @klass             = klass
-      @table             = table
-      @values            = values
-      @loaded            = false
-      @default_scoped    = false
+      @klass  = klass
+      @table  = table
+      @values = values
+      @loaded = false
     end
 
     def initialize_copy(other)
@@ -313,7 +310,7 @@ module ActiveRecord
       stmt.table(table)
       stmt.key = table[primary_key]
 
-      if with_default_scope.joins_values.any?
+      if joins_values.any?
         @klass.connection.join_to_update(stmt, arel)
       else
         stmt.take(arel.limit)
@@ -438,7 +435,7 @@ module ActiveRecord
         stmt = Arel::DeleteManager.new(arel.engine)
         stmt.from(table)
 
-        if with_default_scope.joins_values.any?
+        if joins_values.any?
           @klass.connection.join_to_delete(stmt, arel, table[primary_key])
         else
           stmt.wheres = arel.constraints
@@ -512,12 +509,11 @@ module ActiveRecord
     #   User.where(name: 'Oscar').where_values_hash
     #   # => {name: "Oscar"}
     def where_values_hash
-      scope = with_default_scope
-      equalities = scope.where_values.grep(Arel::Nodes::Equality).find_all { |node|
+      equalities = where_values.grep(Arel::Nodes::Equality).find_all { |node|
         node.left.relation.name == table_name
       }
 
-      binds = Hash[scope.bind_values.find_all(&:first).map { |column, v| [column.name, v] }]
+      binds = Hash[bind_values.find_all(&:first).map { |column, v| [column.name, v] }]
       binds.merge!(Hash[bind_values.find_all(&:first).map { |column, v| [column.name, v] }])
 
       Hash[equalities.map { |where|
@@ -565,16 +561,6 @@ module ActiveRecord
       q.pp(self.to_a)
     end
 
-    def with_default_scope #:nodoc:
-      if default_scoped? && default_scope = klass.send(:build_default_scope)
-        default_scope = default_scope.merge(self)
-        default_scope.default_scoped = false
-        default_scope
-      else
-        self
-      end
-    end
-
     # Returns true if relation is blank.
     def blank?
       to_a.blank?
@@ -594,22 +580,16 @@ module ActiveRecord
     private
 
     def exec_queries
-      default_scoped = with_default_scope
-
-      if default_scoped.equal?(self)
-        @records = eager_loading? ? find_with_associations : @klass.find_by_sql(arel, bind_values)
+      @records = eager_loading? ? find_with_associations : @klass.find_by_sql(arel, bind_values)
 
-        preload = preload_values
-        preload +=  includes_values unless eager_loading?
-        preload.each do |associations|
-          ActiveRecord::Associations::Preloader.new(@records, associations).run
-        end
-
-        @records.each { |record| record.readonly! } if readonly_value
-      else
-        @records = default_scoped.to_a
+      preload = preload_values
+      preload +=  includes_values unless eager_loading?
+      preload.each do |associations|
+        ActiveRecord::Associations::Preloader.new(@records, associations).run
       end
 
+      @records.each { |record| record.readonly! } if readonly_value
+
       @loaded = true
       @records
     end