Override writing behavior

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

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 changing 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.

Examples

Making a field read-only

The same result can be achieved without any code in the field settings ↗.

Making a field read-only can be achieved by passing null as an update handler.

collection.replaceFieldWriting('fullName', null);

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() };

    default:
      throw new Error('Unexpected value');
  }
});

Changing fields in related records

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' } } };
});