Add optional IF NOT EXISTS to add_enum_value

lib/active_record/postgres_enum/postgresql_adapter.rb

@@ -55,8 +55,10 @@ def rename_enum(name, new_name)
         execute "ALTER TYPE #{name} RENAME TO #{new_name}"
       end
 
-      def add_enum_value(name, value, after: nil, before: nil)
-        sql = "ALTER TYPE #{name} ADD VALUE #{quote value}"
+      def add_enum_value(name, value, after: nil, before: nil, if_not_exists: nil)
+        if_not_exists_statement = 'IF NOT EXISTS' if if_not_exists
+
+        sql = "ALTER TYPE #{name} ADD VALUE #{if_not_exists_statement} #{quote value}"
         if after
           sql += " AFTER #{quote after}"
         elsif before

spec/active_record/postgres_enum_spec.rb

@@ -35,6 +35,15 @@
       expect(connection.enums[:foo]).to eq %w(a1 a2 a3)
     end
 
+    it "adds an enum value if not exists" do
+      expect { connection.add_enum_value(:foo, "a1", if_not_exists: true) }.to_not raise_error
+    end
+
+    it "fails to add an enum value if exists" do
+      expect { connection.add_enum_value(:foo, "a1", if_not_exists: false) }.to raise_error
+      expect { connection.add_enum_value(:foo, "a2") }.to raise_error
+    end
+
     it "adds an enum value after a given value" do
       expect { connection.add_enum_value(:foo, "a3", after: "a1") }.to_not raise_error
       expect(connection.enums[:foo]).to eq %w(a1 a3 a2)