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.


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() };
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' } } };