Added where option to add_index to support postgresql partial indices

The `add_index` method now supports a `where` option that receives a
string with the partial index criteria.

    add_index(:accounts, :code, :where => "active")

    Generates

    CREATE INDEX index_accounts_on_code ON accounts(code) WHERE active

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Added support for partial indices to PostgreSQL adapter
+
+    The `add_index` method now supports a `where` option that receives a
+    string with the partial index criteria.
+
+      add_index(:accounts, :code, :where => "active")
+
+      Generates
+
+      CREATE INDEX index_accounts_on_code ON accounts(code) WHERE active
+
+    *Marcelo Silveira*
+
 *   Implemented ActiveRecord::Relation#none method
 
     The `none` method returns a chainable relation with zero records

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -381,9 +381,16 @@ def rename_column(table_name, column_name, new_column_name)
       #
       # Note: mysql doesn't yet support index order (it accepts the syntax but ignores it)
       #
+      # ====== Creating a partial index
+      #  add_index(:accounts, [:branch_id, :party_id], :unique => true, :where => "active")
+      # generates
+      #  CREATE UNIQUE INDEX index_accounts_on_branch_id_and_party_id ON accounts(branch_id, party_id) WHERE active
+      #
+      # Note: only supported by PostgreSQL
+      #
       def add_index(table_name, column_name, options = {})
-        index_name, index_type, index_columns = add_index_options(table_name, column_name, options)
-        execute "CREATE #{index_type} INDEX #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} (#{index_columns})"
+        index_name, index_type, index_columns, index_options = add_index_options(table_name, column_name, options)
+        execute "CREATE #{index_type} INDEX #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} (#{index_columns})#{index_options}"
       end
 
       # Remove the given index from the table.
@@ -581,6 +588,9 @@ def add_index_options(table_name, column_name, options = {})
           if Hash === options # legacy support, since this param was a string
             index_type = options[:unique] ? "UNIQUE" : ""
             index_name = options[:name].to_s if options.key?(:name)
+            if supports_partial_index?
+              index_options = options[:where] ? " WHERE #{options[:where]}" : ""
+            end
           else
             index_type = options
           end
@@ -593,7 +603,7 @@ def add_index_options(table_name, column_name, options = {})
           end
           index_columns = quoted_columns_for_index(column_names, options).join(", ")
 
-          [index_name, index_type, index_columns]
+          [index_name, index_type, index_columns, index_options]
         end
 
         def index_name_for_remove(table_name, options = {})

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -142,6 +142,11 @@ def supports_index_sort_order?
         false
       end
 
+      # Does this adapter support partial indices?
+      def supports_partial_index?
+        false
+      end
+
       # Does this adapter support explain? As of this writing sqlite3,
       # mysql2, and postgresql are the only ones that do.
       def supports_explain?

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -302,6 +302,10 @@ def supports_index_sort_order?
         true
       end
 
+      def supports_partial_index?
+        true
+      end
+
       class StatementPool < ConnectionAdapters::StatementPool
         def initialize(connection, max)
           super

activerecord/test/cases/adapters/postgresql/active_schema_test.rb

@@ -21,6 +21,18 @@ def test_create_database_with_encoding
     assert_equal %(CREATE DATABASE "aimonetti" ENCODING = 'latin1'), create_database(:aimonetti, :encoding => :latin1)
   end
 
+  def test_add_index
+    # add_index calls index_name_exists? which can't work since execute is stubbed
+    ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.send(:define_method, :index_name_exists?) do |*|
+      false
+    end
+
+    expected = %(CREATE UNIQUE INDEX "index_people_on_last_name" ON "people" ("last_name") WHERE state = 'active')
+    assert_equal expected, add_index(:people, :last_name, :unique => true, :where => "state = 'active'")
+
+    ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.send(:remove_method, :index_name_exists?)
+  end
+
   private
     def method_missing(method_symbol, *arguments)
       ActiveRecord::Base.connection.send(method_symbol, *arguments)