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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:
  1. Create a new field containing a foreign key
  2. Make the field filterable for the In operator (required, see Under the hood)
  3. Create a relationship 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'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: customers.map(c => c.id),
      };

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

      return customers.map(record => {
        return rows.find(row => row.group.customer_id === record.id)?.value ?? null;
      });
    },
  });

  // Implement the 'In' operator.
  collection.replaceFieldOperator(
    'lastMessageId',
    'In',
    async (lastMessageIds, context) => {
      const records = await context.dataSource
        .getCollection('messages')
        .list(
          { conditionTree: { field: 'id', operator: 'In', value: lastMessageIds } },
          ['customer_id'],
        );

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

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

​
Connecting collections without a shared identifier

You have 2 Collections both containing users: one from your database, one from your CRM. There is no common id between them, however both have firstName, lastName, and birthDate fields, which taken together are unique enough. 
agent
  .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 => user.map(u => `${u.firstName}/${u.lastName}/${u.birthDate}`),
  });

  // Implement 'In' filtering operator (required)
  collection.replaceFieldOperator('userIdentifier', 'In', values => ({
    aggregator: 'Or',
    conditions: values.map(value => ({
      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',
  });
}