Make implicit order column configurable

When calling ordered finder methods such as +first+ or +last+ without an
explicit order clause, ActiveRecord sorts records by primary key. This
can result in unpredictable and surprising behaviour when the primary
key is not an auto-incrementing integer, for example when it's a UUID.
This change makes it possible to override the column used for implicit
ordering such that +first+ and +last+ will return more predictable
results. For Example:

  class Project < ActiveRecord::Base
    self.implicit_order_column = "created_at"
  end

1 files changed, 16 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/model_schema.rb b/activerecord/lib/active_record/model_schema.rb
index c50a420432..55fc58e339 100644
--- a/activerecord/lib/active_record/model_schema.rb
+++ b/activerecord/lib/active_record/model_schema.rb
@@ -102,6 +102,21 @@ module ActiveRecord
     # If true, the default table name for a Product class will be "products". If false, it would just be "product".
     # See table_name for the full rules on table/class naming. This is true, by default.
 
+    ##
+    # :singleton-method: implicit_order_column
+    # :call-seq: implicit_order_column
+    #
+    # The name of the column 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. Note that using
+    # a non-unique column can result in non-deterministic results.
     included do
       mattr_accessor :primary_key_prefix_type, instance_writer: false
 
@@ -110,6 +125,7 @@ module ActiveRecord
       class_attribute :schema_migrations_table_name, instance_accessor: false, default: "schema_migrations"
       class_attribute :internal_metadata_table_name, instance_accessor: false, default: "ar_internal_metadata"
       class_attribute :pluralize_table_names, instance_writer: false, default: true
+      class_attribute :implicit_order_column, instance_accessor: false
 
       self.protected_environments = ["production"]
       self.inheritance_column = "type"