Generate unique members of a composite key in fixtures

Typically, a fixture's label is hashed into a single unique identifier
that sufficiently represent the respective fixture. In composite key
models, there's still only one label, but a requirement to populate
values for an arbitrary number of columns. Until now, these columns
would all take on the same value, but these may appear odd to an end
user and lead to confusion. This commit uses the index of the column in
the composite key to shift the hash in a deterministic way. In this
case, we bit shift the hashed value index-times and modulo it with the
max. This differentiates components of the key enough that there doesn't
appear to be a correlation.

activerecord/lib/active_record/fixture_set/table_row.rb

@@ -87,7 +87,7 @@ def fill_row_model_attributes
           return unless model_class
           fill_timestamps
           interpolate_label
-          generate_primary_key
+          model_class.composite_primary_key? ? generate_composite_primary_key : generate_primary_key
           resolve_enums
           resolve_sti_reflections
         end
@@ -117,14 +117,28 @@ def interpolate_label
         end
 
         def generate_primary_key
-          # generate a primary key if necessary
-          Array(model_metadata.primary_key_name).each do |pk|
-            next if !model_metadata.has_column?(pk) || @row.include?(pk)
+          pk = model_metadata.primary_key_name
 
+          unless column_defined?(pk)
             @row[pk] = ActiveRecord::FixtureSet.identify(@label, model_metadata.column_type(pk))
           end
         end
 
+        def generate_composite_primary_key
+          id = ActiveRecord::FixtureSet.identify(@label)
+          model_metadata.primary_key_name.each_with_index do |column, index|
+            next if column_defined?(column)
+            raise "Automatic key generation assumes columns of type Integer." unless model_metadata.column_type(column) == :integer
+
+            # Shift label identifier index-#-of-times to differentiate sub-components in deterministic manner.
+            @row[column] = (id << index) % ActiveRecord::FixtureSet::MAX_ID
+          end
+        end
+
+        def column_defined?(col)
+          !model_metadata.has_column?(col) || @row.include?(col)
+        end
+
         def resolve_enums
           reflection_class.defined_enums.each do |name, values|
             if @row.include?(name)

activerecord/test/cases/fixtures_test.rb

@@ -1643,7 +1643,16 @@ def readonly_config
   end
 
   class CompositePkFixturesTest < ActiveRecord::TestCase
-    fixtures :cpk_orders, :cpk_books
+    fixtures :cpk_orders, :cpk_books, :authors
+
+    def test_generates_composite_primary_key_for_partially_filled_fixtures
+      david = authors(:david)
+      david_cpk_book = cpk_books(:cpk_known_author_david_book)
+
+      assert_not_empty(david_cpk_book.id.compact)
+      assert_equal david.id, david_cpk_book.author_id
+      assert_not_nil david_cpk_book.number
+    end
 
     def test_generates_composite_primary_key_ids
       assert_not_empty(cpk_orders(:cpk_groceries_order_1).id.compact)
@@ -1651,5 +1660,9 @@ def test_generates_composite_primary_key_ids
       assert_not_nil(cpk_books(:cpk_great_author_first_book).author_id)
       assert_not_nil(cpk_books(:cpk_great_author_first_book).number)
     end
+
+    def test_generates_composite_primary_key_with_unique_components
+      assert_equal 2, cpk_orders(:cpk_groceries_order_1).id.uniq.size
+    end
   end
 end

activerecord/test/fixtures/cpk_books.yml

@@ -12,3 +12,8 @@ cpk_great_author_second_book:
 cpk_famous_author_first_book:
   title: "Ruby on Rails"
   revision: 1
+
+cpk_known_author_david_book:
+  author_id: 1
+  title: "David's CPK Book"
+  revision: 1