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Forest 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.

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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.

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Making a field read-only

Achieve this without any code in the field settings in the Forest UI.

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Examples

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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 };
});

collection.replace_field_writing('fullName') do |value|
  first_name, last_name = value.split(' ')
  { 'firstName' => first_name, 'lastName' => last_name }
end

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Having specific behavior only for updates

Define 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');
  }
});

collection.replace_field_writing('firstName') do |value, context|
  case context.action
  when 'create'
    { 'firstName' => value, 'firstNameLastEdited' => nil }
  when 'update'
    { 'firstName' => value, 'firstNameLastEdited' => Time.now.iso8601 }
  else
    raise 'Unexpected value'
  end
end
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 } };
});

collection.replace_field_writing('job') do |job, context|
  { 'job' => job, 'portfolio' => { 'title' => job } }
end
If the relationships do not exist, they will be created with the given field values.
Provide another portfolioId to update the relationships and their fields: 
collection.replaceFieldWriting('job', (job, { action }) => {
  return { job, portfolioId: 8, portfolio: { title: job } };
});

collection.replace_field_writing('job') do |job, context|
  { 'job' => job, 'portfolioId' => 8, 'portfolio' => { 'title' => job } }
end
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' } } };
});

collection.replace_field_writing('job') do |job, context|
  { 'job' => job, 'portfolioId' => 8, 'portfolio' => { 'title' => job, 'format' => { 'name' => 'pdf' } } }
end