Override writing behavior

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

Forest Admin allows replacing the default field writing behavior with your own custom logic.

This is useful when you want to change how a given field behaves, but also to make computed fields writable.

How does it work

The replaceFieldWriting function allows to change the behavior of any change by creating a new patch that will be applied to the record.

You should refrain from using handlers that have side effects (to perform error handling, validation, ...) and use hooks instead.

Making a field read-only

Can be achieved without any code in the field settings ↗.


Changing other fields in the same record

In the following example, editing or creating a fullName will update both firstName and lastName fields of the record.

collection.replaceFieldWriting('fullName', value => {
  const [firstName, lastName] = value.split(' ');

  return { firstName, lastName };

Having specific behavior only for updates

You can have different behavior for creations and updates.

In this example, each time the firstName field is edited, we also want to update a timestamp field.

collection.replaceFieldWriting('firstName', async (value, context) => {
  switch (context.action) {
    case 'create':
      return { firstName, firstNameLastEdited: null };

    case 'update':
      return { firstName, firstNameLastEdited: new Date().toISOString() };

      throw new Error('Unexpected value');

Handling relationships inside a replaceFieldWriting will only work for ManyToOne and OneToOne relationships.

In this simple example, we have two collections that are linked together:

  • The Users collection has a job and a portfolioId as foreignKey

  • The Portfolios collection has a title

When the user updates his job field we want also to update the title of the portfolio by the job name.

collection.replaceFieldWriting('job', (job, { action }) => {
  return { job, portfolio: { title: job } };

If the relationships do not exist it will create them with the given field values.

You can also provide another portfolioId to update the relationships and their fields:

collection.replaceFieldWriting('job', (job, { action }) => {
  return { job, portfolioId: 8, portfolio: { title: job } };

Of course, you can chain the relationships. For example, if a portfolio has a one-to-one relationship with the formats collection, you can update it by writing the right path.

collection.replaceFieldWriting('job', (job, { action }) => {
  return { job, portfolioId: 8, portfolio: { title: job, format: { name: 'pdf' } } };

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