Add if_not_exists option to add_enum_value

Adds support to use the PostgreSQL option `IF NOT EXISTS` when adding
enum values via `add_enum_value`.

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Add support for PostgreSQL `IF NOT EXISTS` via the `:if_not_exists` option
+    on the `add_enum_value` method.
+
+    *Ariel Rzezak*
+
 *   When running `db:migrate` on a fresh database, load the database schema before running migrations.
 
     *Andrew Novoselac*

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -584,7 +584,9 @@ module ActiveRecord
       # Add enum value to an existing enum type.
       def add_enum_value(type_name, value, options = {})
         before, after = options.values_at(:before, :after)
-        sql = +"ALTER TYPE #{quote_table_name(type_name)} ADD VALUE '#{value}'"
+        sql = +"ALTER TYPE #{quote_table_name(type_name)} ADD VALUE"
+        sql << " IF NOT EXISTS" if options[:if_not_exists]
+        sql << " '#{value}'"
 
         if before && after
           raise ArgumentError, "Cannot have both :before and :after at the same time"

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

@@ -128,9 +128,14 @@ class PostgresqlEnumTest < ActiveRecord::PostgreSQLTestCase
     @connection.add_enum_value :mood, :nervous, after: :ok
     @connection.add_enum_value :mood, :glad
 
+    assert_nothing_raised do
+      @connection.add_enum_value :mood, :glad, if_not_exists: true
+      @connection.add_enum_value :mood, :curious, if_not_exists: true
+    end
+
     output = dump_table_schema("postgresql_enums")
 
-    assert_includes output, 'create_enum "mood", ["sad", "angry", "ok", "nervous", "happy", "glad"]'
+    assert_includes output, 'create_enum "mood", ["sad", "angry", "ok", "nervous", "happy", "glad", "curious"]'
   end
 
   def test_schema_dump_renamed_enum_value

guides/source/active_record_postgresql.md

@@ -332,6 +332,7 @@ def up
   add_enum_value :article_state, "archived" # will be at the end after published
   add_enum_value :article_state, "in review", before: "published"
   add_enum_value :article_state, "approved", after: "in review"
+  add_enum_value :article_state, "rejected", if_not_exists: true # won't raise an error if the value already exists
 end
 ```