---
title: Using Domain Types in Ent Schema
id: domain
slug: domain-types
---

import Tabs from '@theme/Tabs';
import TabItem from '@theme/TabItem';
import InstallationInstructions from '../components/_installation_instructions.mdx';

PostgreSQL domain types are user-defined data types that extend existing ones, allowing you to add constraints that
restrict the values they can hold. Setting a field type as a domain type enables you to enforce data integrity and
validation rules at the database level.

This guide explains how to define a schema field type as a domain type in your Ent schema and configure the schema migration
to manage both the domains and the Ent schema as a single migration unit using Atlas.

:::info [Atlas Pro Feature](https://atlasgo.io/features#pro-plan)
Atlas support for [Domain Types](https://atlasgo.io/atlas-schema/hcl#domain) is available exclusively to Pro users.
To use this feature, run:
```
atlas login
```
:::

## Install Atlas

<InstallationInstructions />

## Login to Atlas

```shell
$ atlas login a8m
//highlight-next-line-info
You are now connected to "a8m" on Atlas Cloud.
```

## Composite Schema

An `ent/schema` package is mostly used for defining Ent types (objects), their fields, edges and logic. Domain types,
or any other database objects do not have representation in Ent models - A domain type can be defined once,
and may be used multiple times in different fields and models.

In order to extend our PostgreSQL schema to include both custom domain types and our Ent types, we configure Atlas to
read the state of the schema from a [Composite Schema](https://atlasgo.io/atlas-schema/projects#data-source-composite_schema)
data source. Follow the steps below to configure this for your project:

1\. Create a `schema.sql` that defines the necessary domain type. In the same way, you can configure the domain type in
   [Atlas Schema HCL language](https://atlasgo.io/atlas-schema/hcl-types#domain):

<Tabs>
<TabItem value={"sql"} label={"Using SQL"}>

```sql title="schema.sql"
CREATE DOMAIN us_postal_code AS TEXT
CHECK(
   VALUE ~ '^\d{5}$'
   OR VALUE ~ '^\d{5}-\d{4}$'
);
```

</TabItem>
<TabItem value={"hcl"} label={"Using HCL"}>

```hcl title="schema.hcl"
schema "public" {}

domain "us_postal_code" {
  schema = schema.public
  type   = text
  null   = true
  check "us_postal_code_check" {
    expr = "((VALUE ~ '^\\d{5}$'::text) OR (VALUE ~ '^\\d{5}-\\d{4}$'::text))"
  }
}
```

</TabItem>
</Tabs>

2\. In your Ent schema, define a field that uses the domain type only in PostgreSQL dialect:

```go title="ent/schema/user.go" {5-7}
// Fields of the User.
func (User) Fields() []ent.Field {
	return []ent.Field{
		field.String("postal_code").
			SchemaType(map[string]string{
				dialect.Postgres: "us_postal_code",
			}),
	}
}
```

:::note
In case a schema with custom driver-specific types is used with other databases, Ent falls back to the default type
used by the driver (e.g., "varchar").
:::

3\. Create a simple `atlas.hcl` config file with a `composite_schema` that includes both your custom types defined in
   `schema.sql` and your Ent schema:

```hcl title="atlas.hcl"
data "composite_schema" "app" {
  # Load first custom types first.
  schema "public" {
    url = "file://schema.sql"
  }
  # Second, load the Ent schema.
  schema "public" {
    url = "ent://ent/schema"
  }
}

env "local" {
  src = data.composite_schema.app.url
  dev = "docker://postgres/15/dev?search_path=public"
}
```

## Usage

After setting up our schema, we can get its representation using the `atlas schema inspect` command, generate migrations for
it, apply them to a database, and more. Below are a few commands to get you started with Atlas:

#### Inspect the Schema

The `atlas schema inspect` command is commonly used to inspect databases. However, we can also use it to inspect our
`composite_schema` and print the SQL representation of it:

```shell
atlas schema inspect \
  --env local \
  --url env://src \
  --format '{{ sql . }}'
```

The command above prints the following SQL. Note, the `us_postal_code` domain type is defined in the schema before
its usage in the `postal_code` field:

```sql
-- Create domain type "us_postal_code"
CREATE DOMAIN "us_postal_code" AS text CONSTRAINT "us_postal_code_check" CHECK ((VALUE ~ '^\d{5}$'::text) OR (VALUE ~ '^\d{5}-\d{4}$'::text));
-- Create "users" table
CREATE TABLE "users" ("id" bigint NOT NULL GENERATED BY DEFAULT AS IDENTITY, "postal_code" "us_postal_code" NOT NULL, PRIMARY KEY ("id"));
```

#### Generate Migrations For the Schema

To generate a migration for the schema, run the following command:

```shell
atlas migrate diff \
  --env local
```

Note that a new migration file is created with the following content:

```sql title="migrations/20240712090543.sql"
-- Create domain type "us_postal_code"
CREATE DOMAIN "us_postal_code" AS text CONSTRAINT "us_postal_code_check" CHECK ((VALUE ~ '^\d{5}$'::text) OR (VALUE ~ '^\d{5}-\d{4}$'::text));
-- Create "users" table
CREATE TABLE "users" ("id" bigint NOT NULL GENERATED BY DEFAULT AS IDENTITY, "postal_code" "us_postal_code" NOT NULL, PRIMARY KEY ("id"));
```

#### Apply the Migrations

To apply the migration generated above to a database, run the following command:

```
atlas migrate apply \
  --env local \
  --url "postgres://postgres:pass@localhost:5432/database?search_path=public&sslmode=disable"
```

:::info Apply the Schema Directly on the Database

Sometimes, there is a need to apply the schema directly to the database without generating a migration file. For example,
when experimenting with schema changes, spinning up a database for testing, etc. In such cases, you can use the command
below to apply the schema directly to the database:

```shell
atlas schema apply \
  --env local \
  --url "postgres://postgres:pass@localhost:5432/database?search_path=public&sslmode=disable"
```

:::

The code for this guide can be found in [GitHub](https://github.com/ent/ent/tree/master/examples/domaintypes).