> ## Documentation Index > Fetch the complete documentation index at: https://docs.aiinsurance.io/llms.txt > Use this file to discover all available pages before exploring further. # Add Custom Object Relationship > Links a custom object to a target entity via a relationship field. Use the `fieldRefId` from the [Get Configuration](/api-reference/field-model/custom-objects/get-custom-object-configuration) endpoint. **Cardinality behavior:** - **one_to_one**: Replaces any existing link from this object *and* any existing link to the target. Each side can have at most one link. - **one_to_many**: Replaces any existing link from this object (the "one" side), but the target (the "many" side) can have multiple links. - **many_to_one**: Appends the link. The source (the "many" side) can have multiple links, but existing links to the target from other sources on the "one" side are replaced. - **many_to_many**: Appends the link. Both sides can have multiple links. If the exact same link already exists, the operation is idempotent and returns `created: false`. **Required permission:** `company.fmv1_custom_object:update` ## OpenAPI ````yaml /openapi/generated-external-api.yaml put /api/v1/external/companies/{companyId}/custom-objects/{objectType}/{objectId}/relationships/{fieldRefId}/{targetId} openapi: 3.0.3 info: title: AI Insurance External API description: External API for AI Insurance platform version: 1.0.0 contact: email: support@aiinsurance.io servers: - url: https://app.aiinsurance.io description: Production security: - ApiKeyAuth: [] paths: /api/v1/external/companies/{companyId}/custom-objects/{objectType}/{objectId}/relationships/{fieldRefId}/{targetId}: put: tags: - Field Model Custom Objects summary: Add Custom Object Relationship description: > Links a custom object to a target entity via a relationship field. Use the `fieldRefId` from the [Get Configuration](/api-reference/field-model/custom-objects/get-custom-object-configuration) endpoint. **Cardinality behavior:** - **one_to_one**: Replaces any existing link from this object *and* any existing link to the target. Each side can have at most one link. - **one_to_many**: Replaces any existing link from this object (the "one" side), but the target (the "many" side) can have multiple links. - **many_to_one**: Appends the link. The source (the "many" side) can have multiple links, but existing links to the target from other sources on the "one" side are replaced. - **many_to_many**: Appends the link. Both sides can have multiple links. If the exact same link already exists, the operation is idempotent and returns `created: false`. **Required permission:** `company.fmv1_custom_object:update` operationId: addCustomObjectRelationship parameters: - $ref: '#/components/parameters/companyId' - $ref: '#/components/parameters/objectType' - $ref: '#/components/parameters/objectId' - $ref: '#/components/parameters/fieldRefId' - $ref: '#/components/parameters/targetId' responses: '200': description: Relationship added (or already existed) content: application/json: schema: type: object properties: objectId: type: string format: uuid description: The source custom object ID fieldRefId: type: string description: The relationship field reference ID targetId: type: string format: uuid description: The target entity ID created: type: boolean description: | `true` if a new link was created, `false` if the exact link already existed. examples: created: summary: New relationship created value: objectId: 550e8400-e29b-41d4-a716-446655440020 fieldRefId: ownerExposure targetId: 550e8400-e29b-41d4-a716-446655440200 created: true alreadyExists: summary: Relationship already existed value: objectId: 550e8400-e29b-41d4-a716-446655440020 fieldRefId: ownerExposure targetId: 550e8400-e29b-41d4-a716-446655440200 created: false '401': $ref: '#/components/responses/Unauthorized' '403': $ref: '#/components/responses/Forbidden' '404': description: Source or target entity not found content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' examples: sourceNotFound: summary: Source custom object not found value: error: code: NOT_FOUND message: Custom object not found targetNotFound: summary: Target entity not found value: error: code: NOT_FOUND message: >- Target entity '550e8400-e29b-41d4-a716-446655440999' of type 'exposure' not found '500': $ref: '#/components/responses/InternalServerError' components: parameters: companyId: name: companyId in: path required: true schema: type: string format: uuid description: Company identifier objectType: name: objectType in: path required: true schema: type: string description: >- Custom object type in snake_case format (e.g., `vehicle`, `building_info`) objectId: name: objectId in: path required: true schema: type: string format: uuid description: Custom object identifier fieldRefId: name: fieldRefId in: path required: true schema: type: string description: Relationship field reference ID from the configuration endpoint targetId: name: targetId in: path required: true schema: type: string format: uuid description: Target custom object identifier to link or unlink responses: Unauthorized: description: Unauthorized - Invalid or missing API key content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' examples: missingApiKey: summary: Missing API key value: error: code: UNAUTHORIZED message: Authorization header is required bearerTokenNotAllowed: summary: Bearer token used instead of API key value: error: code: UNAUTHORIZED message: External API endpoints require API key authentication Forbidden: description: Forbidden - Insufficient permissions content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' examples: insufficientPermissions: summary: Insufficient permissions value: error: code: FORBIDDEN message: Insufficient permissions to perform this action InternalServerError: description: Internal Server Error - Unexpected error occurred content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' examples: internalError: summary: Unexpected server error value: error: code: INTERNAL_ERROR message: An unexpected error occurred. Please try again later. schemas: ErrorResponse: type: object description: Standard error response for all external API endpoints required: - error properties: error: type: object required: - code - message properties: code: type: string description: Machine-readable error code example: VALIDATION_ERROR message: type: string description: Human-readable error message example: 'submissionId: Required field is missing' details: type: array description: Additional details for validation errors (field-level errors) items: type: object properties: field: type: string description: The field that caused the error example: submissionId message: type: string description: Description of the field error example: Required field is missing securitySchemes: ApiKeyAuth: type: apiKey in: header name: Authorization description: >- API key authentication. Include your API key in the Authorization header. ````