Remove need for macro instance var

Same as we did for collection, removed the `@macro` instance var
and it is now set in each association. Unfortunately it can't be
left undefined in AssociationReflection so it has to be set there.
For now I am setting it to NotImplementedError since there is no
default macro and it changes based on the reflection type.

activerecord/lib/active_record/reflection.rb

@@ -177,12 +177,6 @@ class MacroReflection < AbstractReflection
       # <tt>has_many :clients</tt> returns <tt>:clients</tt>
       attr_reader :name
 
-      # Returns the macro type.
-      #
-      # <tt>composed_of :balance, class_name: 'Money'</tt> returns <tt>:composed_of</tt>
-      # <tt>has_many :clients</tt> returns <tt>:has_many</tt>
-      attr_reader :macro
-
       attr_reader :scope
 
       # Returns the hash of options used for the macro.
@@ -386,7 +380,7 @@ def scope_chain
         scope ? [[scope]] : [[]]
       end
 
-      alias :source_macro :macro
+      def source_macro; macro; end
 
       def has_inverse?
         inverse_name
@@ -408,6 +402,11 @@ def polymorphic_inverse_of(associated_class)
         end
       end
 
+      # Returns the macro type.
+      #
+      # <tt>has_many :clients</tt> returns <tt>:has_many</tt>
+      def macro; raise NotImplementedError; end
+
       # Returns whether or not this association reflection is for a collection
       # association. Returns +true+ if the +macro+ is either +has_many+ or
       # +has_and_belongs_to_many+, +false+ otherwise.
@@ -578,35 +577,39 @@ def primary_key(klass)
 
     class HasManyReflection < AssociationReflection #:nodoc:
       def initialize(name, scope, options, active_record)
-        @macro = :has_many
         super(name, scope, options, active_record)
       end
 
+      def macro; :has_many; end
+
       def collection?
         true
       end
     end
 
     class HasOneReflection < AssociationReflection #:nodoc:
       def initialize(name, scope, options, active_record)
-        @macro = :has_one
         super(name, scope, options, active_record)
       end
+
+      def macro; :has_one; end
     end
 
     class BelongsToReflection < AssociationReflection #:nodoc:
       def initialize(name, scope, options, active_record)
-        @macro = :belongs_to
         super(name, scope, options, active_record)
       end
+
+      def macro; :belongs_to; end
     end
 
     class HasAndBelongsToManyReflection < AssociationReflection #:nodoc:
       def initialize(name, scope, options, active_record)
-        @macro = :has_and_belongs_to_many
         super
       end
 
+      def macro; :has_and_belongs_to_many; end
+
       def collection?
         true
       end