Computed foreign keys

This is the official documentation of the @forestadmin/agent Node.js agent.

You may want to create a relationship between 2 Collections, but you don't have a foreign key that is ready to use to connect them.

To solve that use case, you should use both computed fields and relationships.

This is done with the following steps:

  • create a new Field containing a foreign key,

  • make the Field filterable for the In operator (see Under the hood as to why this is required),

  • create a relation using it.

We have 2 Collections: Customers and Messages, linked together by a one-to-many relationship.

We want to create a ManyToOne relationship with the last message sent by a given customer.

agent.customizeCollection('customers', collection => {
  // Create foreign key
  collection.addField('lastMessageId', {
    columnType: 'Number',
    dependencies: ['id'],
    getValues: async (customers, context) => {
      // We're using Forest Admin's Query Interface (you can use an ORM or plain SQL)
      const messages = context.dataSource.getCollection('messages');
      const conditionTree = {
        field: 'customer_id',
        operator: 'In',
        value: =>,

      const rows = await messages.aggregate(
        { conditionTree },
        { operation: 'Max', field: 'id', groups: [{ field: 'customer_id' }] },

      return => {
        return rows.find(row => === ?? null;

  // Implement the 'In' operator.
    async (lastMessageIds, context) => {
      const records = await context.dataSource
          { conditionTree: { field: 'id', operator: 'In', value: lastMessageIds } },

      return { field: 'id', operator: 'In', value: => r.customer_id) };

  // Create relationships using the foreign key we just added.
  collection.addManyToOneRelation('lastMessage', 'messages', {
    foreignKey: 'lastMessageId',

Connecting collections without having a shared identifier

You have 2 Collections and both contain users: one comes from your database, and the other one is connected to the CRM that your company uses.

There is no common id between them that can be used to tell Forest Admin how to link them together, however, both Collections have firstName, lastName, and birthDate fields, which taken together, are unique enough.

  .customizeCollection('databaseUsers', createFilterableIdentityField)
  .customizeCollection('crmUsers', createFilterableIdentityField)
  .customizeCollection('databaseUsers', createRelationship)
  .customizeCollection('crmUsers', createInverseRelationship);

 * Concatenate firstname, lastname and birthData to make a unique identifier
 * and ensure that the new field is filterable
function createFilterableIdentityField(collection) {
  // Create foreign key on the collection from the database
  collection.addField('userIdentifier', {
    columnType: 'String',
    dependencies: ['firstName', 'lastName', 'birthDate'],
    getValues: user => => `${u.firstName}/${u.lastName}/${u.birthDate}`),

  // Implement 'In' filtering operator (required)
  collection.replaceFieldOperator('userIdentifier', 'In', values => ({
    aggregator: 'Or',
    conditions: => ({
      aggregator: 'And',
      conditions: [
        { field: 'firstName', operator: 'Equal', value: value.split('/')[0] },
        { field: 'lastName', operator: 'Equal', value: value.split('/')[1] },
        { field: 'birthDate', operator: 'Equal', value: value.split('/')[2] },

/** Create relationship between databaseUsers and crmUsers */
function createRelationship(databaseUsers) {
  databaseUsers.addOneToOneRelation('userFromCrm', 'crmUsers', {
    originKey: 'userIdentifier',
    originKeyTarget: 'userIdentifier',

/** Create relationship between crmUsers and databaseUsers */
function createInverseRelationship(crmUsers) {
  crmUsers.addManyToOneRelation('userFromDatabase', 'databaseUsers', {
    foreignKey: 'userIdentifier',
    foreignKeyTarget: 'userIdentifier',

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