twenty/packages/twenty-docs/developers/extend/apps/data/relations.mdx

---
title: Relations
description: Connect objects together with bidirectional MANY_TO_ONE / ONE_TO_MANY relations.
icon: "diagram-project"
---

Relations connect two objects together. In Twenty, relations are always **bidirectional** — every relation has two sides, and each side is declared as a field that references the other.

| Relation type | Description | Has foreign key? |
|---------------|-------------|------------------|
| `MANY_TO_ONE` | Many records of this object point to one record of the target | Yes (`joinColumnName`) |
| `ONE_TO_MANY` | One record of this object has many records of the target | No (the inverse side) |

## How relations work

Every relation requires **two fields** that reference each other:

1. The **MANY_TO_ONE** side — lives on the object that holds the foreign key.
2. The **ONE_TO_MANY** side — lives on the object that owns the collection.

Both fields use `FieldType.RELATION` and cross-reference each other via `relationTargetFieldMetadataUniversalIdentifier`.

## Example: Post Card has many Recipients

A `PostCard` can be sent to many `PostCardRecipient` records. Each recipient belongs to exactly one post card.

**Step 1: Define the ONE_TO_MANY side on PostCard** (the "one" side):

```ts src/fields/post-card-recipients-on-post-card.field.ts
import { defineField, FieldType, RelationType } from 'twenty-sdk/define';
import { POST_CARD_UNIVERSAL_IDENTIFIER } from '../objects/post-card.object';
import { POST_CARD_RECIPIENT_UNIVERSAL_IDENTIFIER } from '../objects/post-card-recipient.object';

// Export so the other side can reference it
export const POST_CARD_RECIPIENTS_FIELD_ID = 'a1111111-1111-1111-1111-111111111111';
// Import from the other side
import { POST_CARD_FIELD_ID } from './post-card-on-post-card-recipient.field';

export default defineField({
  universalIdentifier: POST_CARD_RECIPIENTS_FIELD_ID,
  objectUniversalIdentifier: POST_CARD_UNIVERSAL_IDENTIFIER,
  type: FieldType.RELATION,
  name: 'postCardRecipients',
  label: 'Post Card Recipients',
  icon: 'IconUsers',
  relationTargetObjectMetadataUniversalIdentifier: POST_CARD_RECIPIENT_UNIVERSAL_IDENTIFIER,
  relationTargetFieldMetadataUniversalIdentifier: POST_CARD_FIELD_ID,
  universalSettings: {
    relationType: RelationType.ONE_TO_MANY,
  },
});
```

**Step 2: Define the MANY_TO_ONE side on PostCardRecipient** (the "many" side — holds the foreign key):

```ts src/fields/post-card-on-post-card-recipient.field.ts
import { defineField, FieldType, RelationType, OnDeleteAction } from 'twenty-sdk/define';
import { POST_CARD_UNIVERSAL_IDENTIFIER } from '../objects/post-card.object';
import { POST_CARD_RECIPIENT_UNIVERSAL_IDENTIFIER } from '../objects/post-card-recipient.object';

// Export so the other side can reference it
export const POST_CARD_FIELD_ID = 'b2222222-2222-2222-2222-222222222222';
// Import from the other side
import { POST_CARD_RECIPIENTS_FIELD_ID } from './post-card-recipients-on-post-card.field';

export default defineField({
  universalIdentifier: POST_CARD_FIELD_ID,
  objectUniversalIdentifier: POST_CARD_RECIPIENT_UNIVERSAL_IDENTIFIER,
  type: FieldType.RELATION,
  name: 'postCard',
  label: 'Post Card',
  icon: 'IconMail',
  relationTargetObjectMetadataUniversalIdentifier: POST_CARD_UNIVERSAL_IDENTIFIER,
  relationTargetFieldMetadataUniversalIdentifier: POST_CARD_RECIPIENTS_FIELD_ID,
  universalSettings: {
    relationType: RelationType.MANY_TO_ONE,
    onDelete: OnDeleteAction.CASCADE,
    joinColumnName: 'postCardId',
  },
});
```

<Note>
**Circular imports:** both relation fields reference each other's `universalIdentifier`. To avoid circular import issues, export your field IDs as named constants from each file and import them in the other. The build system resolves these at compile time.
</Note>

## Relating to standard objects

To create a relation with a built-in Twenty object (Person, Company, etc.), use `STANDARD_OBJECT_UNIVERSAL_IDENTIFIERS`:

```ts src/fields/person-on-self-hosting-user.field.ts
import {
  defineField,
  FieldType,
  RelationType,
  OnDeleteAction,
  STANDARD_OBJECT_UNIVERSAL_IDENTIFIERS,
} from 'twenty-sdk/define';
import { SELF_HOSTING_USER_UNIVERSAL_IDENTIFIER } from '../objects/self-hosting-user.object';

export const PERSON_FIELD_ID = 'c3333333-3333-3333-3333-333333333333';
export const SELF_HOSTING_USER_REVERSE_FIELD_ID = 'd4444444-4444-4444-4444-444444444444';

export default defineField({
  universalIdentifier: PERSON_FIELD_ID,
  objectUniversalIdentifier: SELF_HOSTING_USER_UNIVERSAL_IDENTIFIER,
  type: FieldType.RELATION,
  name: 'person',
  label: 'Person',
  description: 'Person matching with the self hosting user',
  isNullable: true,
  relationTargetObjectMetadataUniversalIdentifier:
    STANDARD_OBJECT_UNIVERSAL_IDENTIFIERS.person.universalIdentifier,
  relationTargetFieldMetadataUniversalIdentifier: SELF_HOSTING_USER_REVERSE_FIELD_ID,
  universalSettings: {
    relationType: RelationType.MANY_TO_ONE,
    onDelete: OnDeleteAction.SET_NULL,
    joinColumnName: 'personId',
  },
});
```

## Relation field properties

| Property | Required | Description |
|----------|----------|-------------|
| `type` | Yes | Must be `FieldType.RELATION` |
| `relationTargetObjectMetadataUniversalIdentifier` | Yes | The `universalIdentifier` of the target object |
| `relationTargetFieldMetadataUniversalIdentifier` | Yes | The `universalIdentifier` of the matching field on the target object |
| `universalSettings.relationType` | Yes | `RelationType.MANY_TO_ONE` or `RelationType.ONE_TO_MANY` |
| `universalSettings.onDelete` | MANY_TO_ONE only | What happens when the referenced record is deleted: `CASCADE`, `SET_NULL`, `RESTRICT`, or `NO_ACTION` |
| `universalSettings.joinColumnName` | MANY_TO_ONE only | Database column name for the foreign key (e.g., `postCardId`) |

## Inline relation fields

You can also declare a relation directly inside [`defineObject`](/developers/extend/apps/data/objects). When inline, omit `objectUniversalIdentifier` — it's inherited from the parent object:

```ts
export default defineObject({
  universalIdentifier: '...',
  nameSingular: 'postCardRecipient',
  // ...
  fields: [
    {
      universalIdentifier: POST_CARD_FIELD_ID,
      type: FieldType.RELATION,
      name: 'postCard',
      label: 'Post Card',
      relationTargetObjectMetadataUniversalIdentifier: POST_CARD_UNIVERSAL_IDENTIFIER,
      relationTargetFieldMetadataUniversalIdentifier: POST_CARD_RECIPIENTS_FIELD_ID,
      universalSettings: {
        relationType: RelationType.MANY_TO_ONE,
        onDelete: OnDeleteAction.CASCADE,
        joinColumnName: 'postCardId',
      },
    },
    // … other fields
  ],
});
```