> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sqd.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Schema file and codegen

> Introduction to the Squid SDK schema.graphql file and codegen tool — define entities, run sqd codegen, and generate TypeORM models for storage.

The schema file `schema.graphql` uses a GraphQL dialect to model the target entities and entity relations. The tooling around the schema file is then used to:

* Generate TypeORM entities (with `squid-typeorm-codegen(1)`, see below)
* Generate the database schema from the TypeORM entities (see [db migrations](/en/sdk/squid-sdk/resources/persisting-data/typeorm))
* Optionally, the schema can be used to present the target data with a [GraphQL API](/en/sdk/squid-sdk/resources/serving-graphql).

The schema file format is loosely compatible with the [subgraph schema](https://thegraph.com/docs/en/developing/creating-a-subgraph/) file, see [Migrate from subgraph](/en/sdk/squid-sdk/resources/migrate/migrate-subgraph) section for details.

## TypeORM codegen

The [`squid-typeorm-codegen(1)`](https://github.com/subsquid/squid-sdk/tree/master/typeorm/typeorm-codegen) tool is used to generate [TypeORM entity](https://typeorm.io/) classes from the schema defined in `schema.graphql`. Invoke it with

```bash theme={"system"}
npx squid-typeorm-codegen
```

By default the entity classes are generated in `src/model/generated`.

### Example

A `Foo` entity defined in the schema file:

```graphql title="schema.graphql" theme={"system"}
type Foo @entity {
  id: ID!
  bar: String
  baz: BigInt!
}
```

The generated `Foo` entity with TypeORM decorators:

```ts title="src/model/generated/foo.ts" theme={"system"}
import {Entity as Entity_, Column as Column_, PrimaryColumn as PrimaryColumn_} from "typeorm"
import * as marshal from "./marshal"

@Entity_()
export class Foo {
    constructor(props?: Partial<Foo>) {
        Object.assign(this, props)
    }

    @PrimaryColumn_()
    id!: string

    @Column_("text", {nullable: true})
    bar!: string | undefined | null

    @Column_("numeric", {transformer: marshal.bigintTransformer, nullable: false})
    baz!: bigint
}
```