Add support for if_not_exists to indexes

When we try to create a table which already exists which also adds
indexes, then the `if_not_exists` option passed to `create_table` is
not extended to indexes. So the migration results into an error if the
table and indexes already exist.
This change extends the `if_not_exists` support to `add_index` so that
if the migration tries to create a table which also has existing
indexes, error won't be raised.
Also as a side-effect individual `add_index` calls will also accept
`if_not_exists` option and respect it henceforth.

[Prathamesh Sonpatki, Justin George]

activerecord/CHANGELOG.md

@@ -1,3 +1,20 @@
+*   Add support for `if_not_exists` option for adding index.
+
+    The `add_index` method respects `if_not_exists` option. If it is set to true
+    index won't be added.
+
+    Usage:
+
+    ```
+      add_index :users, :account_id, if_not_exists: true
+    ```
+
+    The `if_not_exists` option passed to `create_table` also gets propogated to indexes
+    created within that migration so that if table and its indexes exist then there is no
+    attempt to create them again.
+
+    *Prathamesh Sonpatki*
+
 *   Add `ActiveRecord::Base#previously_new_record?` to show if a record was new before the last save.
 
     *Tom Ward*

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

@@ -317,7 +317,7 @@ module ActiveRecord
 
         unless supports_indexes_in_create?
           td.indexes.each do |column_name, index_options|
-            add_index(table_name, column_name, index_options)
+            add_index(table_name, column_name, index_options.merge!(if_not_exists: td.if_not_exists))
           end
         end
 
@@ -706,6 +706,16 @@ module ActiveRecord
       #
       #   CREATE INDEX suppliers_name_index ON suppliers(name)
       #
+      # ====== Creating a index which already exists
+      #
+      #   add_index(:suppliers, :name, if_not_exists: true)
+      #
+      # generates:
+      #
+      #   CREATE INDEX IF NOT EXISTS suppliers_name_index ON suppliers(name)
+      #
+      # Note: Not supported by MySQL.
+      #
       # ====== Creating a unique index
       #
       #   add_index(:accounts, [:branch_id, :party_id], unique: true)
@@ -805,8 +815,8 @@ module ActiveRecord
       #
       # For more information see the {"Transactional Migrations" section}[rdoc-ref:Migration].
       def add_index(table_name, column_name, options = {})
-        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}"
+        index_name, index_type, index_columns, index_if_not_exists_clause, index_options = add_index_options(table_name, column_name, **options)
+        execute "CREATE #{index_type} #{index_if_not_exists_clause} #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} (#{index_columns})#{index_options}"
       end
 
       # Removes the given index from the table.
@@ -1200,7 +1210,7 @@ module ActiveRecord
       def add_index_options(table_name, column_name, comment: nil, **options) # :nodoc:
         column_names = index_column_names(column_name)
 
-        options.assert_valid_keys(:unique, :order, :name, :where, :length, :internal, :using, :algorithm, :type, :opclass)
+        options.assert_valid_keys(:unique, :order, :name, :where, :length, :internal, :using, :algorithm, :type, :opclass, :if_not_exists)
 
         index_type = options[:type].to_s if options.key?(:type)
         index_type ||= options[:unique] ? "UNIQUE" : ""
@@ -1219,11 +1229,13 @@ module ActiveRecord
           index_options = options[:where] ? " WHERE #{options[:where]}" : ""
         end
 
+        if_not_exists_index_clause = options[:if_not_exists] ? "INDEX IF NOT EXISTS" : "INDEX"
+
         validate_index_length!(table_name, index_name, options.fetch(:internal, false))
 
         index_columns = quoted_columns_for_index(column_names, **options).join(", ")
 
-        [index_name, index_type, index_columns, index_options, algorithm, using, comment]
+        [index_name, index_type, index_columns, if_not_exists_index_clause, index_options, algorithm, using, comment]
       end
 
       def options_include_default?(options)

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -360,7 +360,9 @@ module ActiveRecord
       end
 
       def add_index(table_name, column_name, options = {}) #:nodoc:
-        index_name, index_type, index_columns, _, index_algorithm, index_using, comment = add_index_options(table_name, column_name, **options)
+        return if options[:if_not_exists] && index_exists?(table_name, column_name, options)
+
+        index_name, index_type, index_columns, _, _, index_algorithm, index_using, comment = add_index_options(table_name, column_name, **options)
         sql = +"CREATE #{index_type} INDEX #{quote_column_name(index_name)} #{index_using} ON #{quote_table_name(table_name)} (#{index_columns}) #{index_algorithm}"
         execute add_sql_comment!(sql, comment)
       end
@@ -673,7 +675,7 @@ module ActiveRecord
         end
 
         def add_index_for_alter(table_name, column_name, options = {})
-          index_name, index_type, index_columns, _, index_algorithm, index_using = add_index_options(table_name, column_name, **options)
+          index_name, index_type, index_columns, _, _, index_algorithm, index_using = add_index_options(table_name, column_name, **options)
           index_algorithm[0, 0] = ", " if index_algorithm.present?
           "ADD #{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})#{index_algorithm}"
         end

activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb

@@ -62,7 +62,7 @@ module ActiveRecord
           end
 
           def index_in_create(table_name, column_name, options)
-            index_name, index_type, index_columns, _, _, index_using, comment = @conn.add_index_options(table_name, column_name, **options)
+            index_name, index_type, index_columns, _, _, _, index_using, comment = @conn.add_index_options(table_name, column_name, **options)
             add_sql_comment!((+"#{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})"), comment)
           end
       end

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

@@ -442,8 +442,8 @@ module ActiveRecord
         end
 
         def add_index(table_name, column_name, options = {}) #:nodoc:
-          index_name, index_type, index_columns_and_opclasses, index_options, index_algorithm, index_using, comment = add_index_options(table_name, column_name, **options)
-          execute("CREATE #{index_type} INDEX #{index_algorithm} #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} #{index_using} (#{index_columns_and_opclasses})#{index_options}").tap do
+          index_name, index_type, index_columns_and_opclasses, index_if_not_exists_clause, index_options, index_algorithm, index_using, comment = add_index_options(table_name, column_name, **options)
+          execute("CREATE #{index_type} #{index_if_not_exists_clause} #{index_algorithm} #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} #{index_using} (#{index_columns_and_opclasses})#{index_options}").tap do
             execute "COMMENT ON INDEX #{quote_column_name(index_name)} IS #{quote(comment)}" if comment
           end
         end

activerecord/test/cases/adapters/mysql2/active_schema_test.rb

@@ -61,6 +61,15 @@ class Mysql2ActiveSchemaTest < ActiveRecord::Mysql2TestCase
     expected = "CREATE  INDEX `index_people_on_last_name` USING btree ON `people` (`last_name`(10)) ALGORITHM = COPY"
     assert_equal expected, add_index(:people, :last_name, length: 10, using: :btree, algorithm: :copy)
 
+    with_real_execute do
+      add_index(:people, :first_name)
+      assert index_exists?(:people, :first_name)
+
+      assert_nothing_raised do
+        add_index(:people, :first_name, if_not_exists: true)
+      end
+    end
+
     assert_raise ArgumentError do
       add_index(:people, :last_name, algorithm: :coyp)
     end

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

@@ -67,6 +67,9 @@ class PostgresqlActiveSchemaTest < ActiveRecord::PostgreSQLTestCase
     expected = %(CREATE  INDEX  "index_people_on_last_name" ON "people"  ("last_name" NULLS FIRST))
     assert_equal expected, add_index(:people, :last_name, order: "NULLS FIRST")
 
+    expected = %(CREATE  INDEX IF NOT EXISTS  "index_people_on_last_name" ON "people"  ("last_name"))
+    assert_equal expected, add_index(:people, :last_name, if_not_exists: true)
+
     assert_raise ArgumentError do
       add_index(:people, :last_name, algorithm: :copy)
     end

activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb

@@ -355,6 +355,16 @@ module ActiveRecord
         end
       end
 
+      def test_index_with_if_not_exists
+        with_example_table do
+          @conn.add_index "ex", "id"
+
+          assert_nothing_raised do
+            @conn.add_index "ex", "id", if_not_exists: true
+          end
+        end
+      end
+
       def test_non_unique_index
         with_example_table do
           @conn.add_index "ex", "id", name: "fun"

activerecord/test/cases/migration/index_test.rb

@@ -72,6 +72,14 @@ module ActiveRecord
         connection.add_index(table_name, "foo", name: good_index_name)
       end
 
+      def test_add_index_which_already_exists_does_not_raise_error
+        connection.add_index(table_name, "foo", if_not_exists: true)
+
+        assert_nothing_raised do
+          connection.add_index(table_name, "foo", if_not_exists: true)
+        end
+      end
+
       def test_internal_index_with_name_matching_database_limit
         good_index_name = "x" * connection.index_name_length
         connection.add_index(table_name, "foo", name: good_index_name, internal: true)

activerecord/test/cases/migration_test.rb

@@ -155,6 +155,23 @@ class MigrationTest < ActiveRecord::TestCase
     connection.drop_table :testings, if_exists: true
   end
 
+  def test_create_table_with_indexes_and_if_not_exists_true
+    connection = Person.connection
+    connection.create_table :testings, force: true do |t|
+      t.references :people
+      t.string :foo
+    end
+
+    assert_nothing_raised do
+      connection.create_table :testings, if_not_exists: true do |t|
+        t.references :people
+        t.string :foo
+      end
+    end
+  ensure
+    connection.drop_table :testings, if_exists: true
+  end
+
   def test_create_table_with_force_true_does_not_drop_nonexisting_table
     # using a copy as we need the drop_table method to
     # continue to work for the ensure block of the test