feat: implement generic many-to-many junction relation support (#16820)

## Overview

This PR implements **generic many-to-many relation support** through
junction tables (also known as associative entities or join tables).
This replaces the need for hardcoded taskTarget/noteTarget logic and
provides a flexible foundation for modeling complex entity
relationships.

## Architecture

### Data Model

Many-to-many relationships are implemented using a **junction object
pattern**:

```
┌─────────┐       ┌──────────────────┐       ┌─────────┐
│  Pet    │──────>│   PetRocket      │<──────│ Rocket  │
│         │ 1:N   │  (junction)      │  N:1  │         │
│ rockets ├───────┤ pet    : Pet     ├───────┤         │
└─────────┘       │ rocket : Rocket  │       └─────────┘
                  └──────────────────┘
```

The junction object (PetRocket) has:
- A `MANY_TO_ONE` relation to **Pet** (the source)
- A `MANY_TO_ONE` relation to **Rocket** (the target)

The source object (Pet) has a `ONE_TO_MANY` relation pointing to the
junction, with **field settings** that specify which target field to
follow.

### Field Settings Schema

Junction configuration is stored in `FieldMetadataRelationSettings`:

```typescript
{
  relationType: "ONE_TO_MANY",
  // Points to the target field on the junction object
  junctionTargetFieldId?: string;      // For regular relations
  junctionTargetMorphId?: string;      // For polymorphic relations
}
```

**Two configuration modes:**
1. **`junctionTargetFieldId`** - References a specific `RELATION` field
on the junction
2. **`junctionTargetMorphId`** - References a `morphId` group for
polymorphic targets (e.g., link to Person OR Company)

### GraphQL Query Generation

When a junction relation is detected, the GraphQL fields are generated
to fetch the nested target:

```graphql
query GetPetWithRockets {
  pet(id: "...") {
    rockets {           # ONE_TO_MANY to junction
      id
      rocket {          # Target field on junction
        id
        name
        __typename
      }
    }
  }
}
```

For polymorphic junction targets:
```graphql
caretakerPerson { id, name }
caretakerCompany { id, name }
```

## Frontend Architecture

### Display Flow

1. **Detection**: `hasJunctionConfig()` checks if field has junction
settings
2. **Config Resolution**: `getJunctionConfig()` resolves junction object
metadata and target fields
3. **Record Extraction**: `extractTargetRecordsFromJunction()` extracts
target records from junction records
4. **Rendering**: Target records displayed as chips (not junction
records)

### Edit Flow

1. **Picker Opening**: Initializes the multi-record picker with:
   - Searchable object types (derived from junction target fields)
   - Pre-selected items (extracted from existing junction records)
   
2. **Selection Handling**: Manages create/delete of junction records:
   - **Select**: Creates new junction record with source + target IDs
   - **Deselect**: Finds and deletes the junction record
- **Optimistic Updates**: Manually updates Recoil store before API call

### Key Trade-offs

| Decision | Trade-off |
|----------|-----------|
| Junction records managed manually | More control over optimistic
updates, but requires manual cache management |
| Settings stored per-field | Flexible (same junction can power
different views), but requires UI to configure |
| Polymorphic via morphId groups | Supports N target types, but adds
query complexity |
| Feature flag gated | Safe rollout, but requires flag management |

## Backend Changes

- **Validation**: Junction target field must exist and be a valid
`MANY_TO_ONE` relation
- **Settings**: Extended `FieldMetadataRelationSettings` type with
junction fields
- **Dev Seeder**: Added sample junction objects (PetRocket,
EmploymentHistory, PetCareAgreement) for testing

## How to Test

1. Enable the `IS_JUNCTION_RELATIONS_ENABLED` feature flag
2. Create objects with junction pattern (Pet → PetRocket → Rocket)
3. Configure the junction target in field settings (advanced mode)
4. Verify:
- Display shows target objects (Rockets), not junction records
(PetRockets)
   - Picker allows selecting/deselecting targets
   - Changes persist correctly



https://github.com/user-attachments/assets/d04f057a-228c-4de8-af48-76bb2d72cac1

---------

Co-authored-by: Charles Bochet <charles@twenty.com>

packages/twenty-server/src/engine/metadata-modules/object-metadata/dtos/create-object.input.ts

@@ -67,6 +67,11 @@ export class CreateObjectInput {
   @HideField()
   universalIdentifier?: string;
 
+  @IsBoolean()
+  @IsOptional()
+  @Field({ nullable: true })
+  skipNameField?: boolean;
+
   @IsBoolean()
   @IsOptional()
   @Field({ nullable: true })

packages/twenty-server/src/engine/metadata-modules/object-metadata/object-metadata.service.ts

@@ -375,6 +375,8 @@ export class ObjectMetadataService extends TypeOrmQueryService<ObjectMetadataEnt
         objectFlatFieldMetadatas: flatFieldMetadataToCreateOnObject,
         viewId: flatDefaultViewToCreate.id,
         workspaceId,
+        labelIdentifierFieldMetadataId:
+          flatObjectMetadataToCreate.labelIdentifierFieldMetadataId,
       });
 
     const validateAndBuildResult =
@@ -483,14 +485,21 @@ export class ObjectMetadataService extends TypeOrmQueryService<ObjectMetadataEnt
     viewId,
     workspaceId,
     workspaceCustomApplicationId,
+    labelIdentifierFieldMetadataId,
   }: {
     workspaceCustomApplicationId: string;
     objectFlatFieldMetadatas: FlatFieldMetadata[];
     viewId: string;
     workspaceId: string;
+    labelIdentifierFieldMetadataId: string | null;
   }) {
     const defaultViewFields = objectFlatFieldMetadatas
-      .filter((field) => field.name !== 'id' && field.name !== 'deletedAt')
+      .filter(
+        (field) =>
+          field.name !== 'deletedAt' &&
+          // Include 'id' only if it's the label identifier (e.g., for junction tables)
+          (field.name !== 'id' || field.id === labelIdentifierFieldMetadataId),
+      )
       .map((field, index) =>
         fromCreateViewFieldInputToFlatViewFieldToCreate({
           createViewFieldInput: {

packages/twenty-server/src/engine/metadata-modules/object-metadata/utils/build-default-flat-field-metadatas-for-custom-object.util.ts

@@ -17,6 +17,7 @@ type BuildDefaultFlatFieldMetadataForCustomObjectArgs = {
   flatObjectMetadata: NonNullableRequired<
     Pick<FlatObjectMetadata, 'id' | 'applicationId'>
   >;
+  skipNameField?: boolean;
 };
 
 export type DefaultFlatFieldForCustomObjectMaps = ReturnType<
@@ -26,6 +27,7 @@ export type DefaultFlatFieldForCustomObjectMaps = ReturnType<
 export const buildDefaultFlatFieldMetadatasForCustomObject = ({
   workspaceId,
   flatObjectMetadata: { id: objectMetadataId, applicationId },
+  skipNameField = false,
 }: BuildDefaultFlatFieldMetadataForCustomObjectArgs) => {
   const createdAt = new Date().toISOString();
   const idFieldId = v4();
@@ -65,42 +67,45 @@ export const buildDefaultFlatFieldMetadatasForCustomObject = ({
     applicationId,
   };
 
-  const nameFieldId = v4();
-  const nameField: FlatFieldMetadata<FieldMetadataType.TEXT> = {
-    type: FieldMetadataType.TEXT,
-    id: nameFieldId,
-    viewFieldIds: [],
-    mainGroupByFieldMetadataViewIds: [],
-    kanbanAggregateOperationViewIds: [],
-    calendarViewIds: [],
-    isLabelSyncedWithName: false,
-    isUnique: false,
-    objectMetadataId,
-    universalIdentifier: nameFieldId,
-    workspaceId,
-    standardId: CUSTOM_OBJECT_STANDARD_FIELD_IDS.name,
-    name: 'name',
-    label: 'Name',
-    icon: 'IconAbc',
-    description: 'Name',
-    isNullable: true,
-    isActive: true,
-    isCustom: false,
-    isSystem: false,
-    isUIReadOnly: false,
-    defaultValue: null,
-    viewFilterIds: [],
+  const nameFieldId = skipNameField ? null : v4();
+  const nameField: FlatFieldMetadata<FieldMetadataType.TEXT> | null =
+    skipNameField
+      ? null
+      : {
+          type: FieldMetadataType.TEXT,
+          id: nameFieldId!,
+          viewFieldIds: [],
+          mainGroupByFieldMetadataViewIds: [],
+          kanbanAggregateOperationViewIds: [],
+          calendarViewIds: [],
+          isLabelSyncedWithName: false,
+          isUnique: false,
+          objectMetadataId,
+          universalIdentifier: nameFieldId!,
+          workspaceId,
+          standardId: CUSTOM_OBJECT_STANDARD_FIELD_IDS.name,
+          name: 'name',
+          label: 'Name',
+          icon: 'IconAbc',
+          description: 'Name',
+          isNullable: true,
+          isActive: true,
+          isCustom: false,
+          isSystem: false,
+          isUIReadOnly: false,
+          defaultValue: null,
+          viewFilterIds: [],
 
-    createdAt,
-    updatedAt: createdAt,
-    options: null,
-    standardOverrides: null,
-    relationTargetFieldMetadataId: null,
-    relationTargetObjectMetadataId: null,
-    settings: null,
-    morphId: null,
-    applicationId,
-  };
+          createdAt,
+          updatedAt: createdAt,
+          options: null,
+          standardOverrides: null,
+          relationTargetFieldMetadataId: null,
+          relationTargetObjectMetadataId: null,
+          settings: null,
+          morphId: null,
+          applicationId,
+        };
 
   const createdAtFieldId = v4();
   const createdAtField: FlatFieldMetadata<FieldMetadataType.DATE_TIME> = {
@@ -355,7 +360,9 @@ export const buildDefaultFlatFieldMetadatasForCustomObject = ({
     relationTargetFieldMetadataId: null,
     relationTargetObjectMetadataId: null,
     settings: {
-      asExpression: getTsVectorColumnExpressionFromFields([nameField]),
+      asExpression: getTsVectorColumnExpressionFromFields(
+        nameField ? [nameField] : [],
+      ),
       generatedType: 'STORED',
     },
     morphId: null,
@@ -365,7 +372,7 @@ export const buildDefaultFlatFieldMetadatasForCustomObject = ({
   return {
     fields: {
       idField,
-      nameField,
+      ...(nameField && { nameField }),
       createdAtField,
       updatedAtField,
       updatedByField,