activerecord: Don't always append primary keys etc. to order conditions

- If `nil` is the last element of an array passed to
  `implicit_order_column`, do not append the primary key
  or the query constraints. For example,
  `self.implicit_order_column = ["author_name", nil]` will
  generate `ORDER BY author_name DESC` and not
  `ORDER BY author_name DESC, id DESC` (with
  or without a LIMIT).

- There wasn't a test for `implicit_order_column` supporting
  arrays for ordering by multiple columns, so I added one.

- The reasoning is that in some cases, like database sharding, the
  primary key is not the best column to order by for performance
  reasons and thus users should have the choice of whether to append
  it by default.

Author: issyl0

activerecord/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   Allow bypassing primary key/constraint addition in `implicit_order_column`
+
+    When specifying multiple columns in an array for `implicit_order_column`, adding
+    `nil` as the last element will prevent appending the primary key to order
+    conditions. This allows more precise control of indexes used by
+    generated queries. It should be noted that this feature does introduce the risk
+    of API misbehavior if the specified columns are not fully unique.
+
+    *Issy Long*
+
 *   Allow setting the `schema_format` via database configuration.
 
     ```

activerecord/lib/active_record/model_schema.rb

@@ -113,17 +113,19 @@ module ModelSchema
     # :singleton-method: implicit_order_column
     # :call-seq: implicit_order_column
     #
-    # The name of the column records are ordered by if no explicit order clause
+    # The name of the column(s) records are ordered by if no explicit order clause
     # is used during an ordered finder call. If not set the primary key is used.
 
     ##
     # :singleton-method: implicit_order_column=
     # :call-seq: implicit_order_column=(column_name)
     #
-    # Sets the column to sort records by when no explicit order clause is used
-    # during an ordered finder call. Useful when the primary key is not an
-    # auto-incrementing integer, for example when it's a UUID. Records are subsorted
-    # by the primary key if it exists to ensure deterministic results.
+    # Sets the column(s) to sort records by when no explicit order clause is used
+    # during an ordered finder call. Useful for models where the primary key isn't an
+    # auto-incrementing integer (such as UUID).
+    #
+    # By default, records are subsorted by primary key to ensure deterministic results.
+    # To disable this subsort behavior, set `implicit_order_column` to `["column_name", nil]`.
 
     ##
     # :singleton-method: immutable_strings_by_default=

activerecord/lib/active_record/relation/finder_methods.rb

@@ -646,16 +646,13 @@ def ordered_relation
       end
 
       def _order_columns
-        oc = []
+        columns = Array(model.implicit_order_column)
 
-        oc << model.implicit_order_column if model.implicit_order_column
-        oc << model.query_constraints_list if model.query_constraints_list
+        return columns.compact if columns.length.positive? && columns.last.nil?
 
-        if model.primary_key && model.query_constraints_list.nil?
-          oc << model.primary_key
-        end
+        columns += Array(model.query_constraints_list || model.primary_key)
 
-        oc.flatten.uniq.compact
+        columns.uniq.compact
       end
   end
 end

activerecord/test/cases/finder_test.rb

@@ -1106,7 +1106,7 @@ def test_first_have_determined_order_by_default
     assert_equal expected, clients.order(nil).first(2)
   end
 
-  def test_implicit_order_column_is_configurable
+  def test_implicit_order_column_is_configurable_with_a_single_value
     old_implicit_order_column = Topic.implicit_order_column
     Topic.implicit_order_column = "title"
 
@@ -1120,6 +1120,28 @@ def test_implicit_order_column_is_configurable
     Topic.implicit_order_column = old_implicit_order_column
   end
 
+  def test_implicit_order_column_is_configurable_with_multiple_values
+    old_implicit_order_column = Topic.implicit_order_column
+    Topic.implicit_order_column = ["title", "author_name"]
+
+    assert_queries_match(/ORDER BY #{Regexp.escape(quote_table_name("topics.title"))} DESC, #{Regexp.escape(quote_table_name("topics.author_name"))} DESC, #{Regexp.escape(quote_table_name("topics.id"))} DESC LIMIT/i) {
+      Topic.last
+    }
+  ensure
+    Topic.implicit_order_column = old_implicit_order_column
+  end
+
+  def test_ordering_does_not_append_primary_keys_or_query_constraints_if_passed_an_implicit_order_column_array_ending_in_nil
+    old_implicit_order_column = Topic.implicit_order_column
+    Topic.implicit_order_column = ["author_name", nil]
+
+    assert_queries_match(/ORDER BY #{Regexp.escape(quote_table_name("topics.author_name"))} DESC LIMIT/i) {
+      Topic.last
+    }
+  ensure
+    Topic.implicit_order_column = old_implicit_order_column
+  end
+
   def test_implicit_order_set_to_primary_key
     old_implicit_order_column = Topic.implicit_order_column
     Topic.implicit_order_column = "id"