# Override writing behavior

{% hint style="success" %}
This is the official documentation of the `agent_ruby` Ruby agent.
{% endhint %}

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](/developer-guide-agents-ruby/agent-customization/fields/computed.md) writable.

### How does it work

The `replace_field_writing` 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](/developer-guide-agents-ruby/agent-customization/hooks.md).

### Making a field read-only

{% hint style="info" %}
Can be achieved without any code [in the field settings](https://docs.forestadmin.com/user-guide/collections/customize-your-fields#basic-settings).
{% endhint %}

### Examples

#### 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.

#### Traditional Syntax

Using full context methods with `customize_collection`:

```ruby
@create_agent.customize_collection('customer') do |collection|
  collection.replace_field_writing('fullName') do |value|
    first_name, last_name = value.split(' ')
    {
      'firstName' => first_name,
      'lastName' => last_name
    }
  end
end
```

#### DSL Syntax

Using simplified DSL with `collection`:

```ruby
@create_agent.collection :customer do |collection|
  collection.replace_writing :fullName do |value|
    first_name, last_name = value.split(' ')
    {
      'firstName' => first_name,
      'lastName' => last_name
    }
  end
end
```

#### 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.

#### Traditional Syntax

Using full context methods with `customize_collection`:

```ruby
@create_agent.customize_collection('customer') do |collection|
  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
end
```

#### DSL Syntax

Using simplified DSL with `collection`:

```ruby
@create_agent.collection :customer do |collection|
  collection.replace_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
end
```

#### Changing fields in related records

{% hint style="info" %}
Handling relationships inside a `replaceFieldWriting``replace_field_writing` will only work for `ManyToOne` and `OneToOne` relationships.
{% endhint %}

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.

#### Traditional Syntax

Using full context methods with `customize_collection`:

```ruby
@create_agent.customize_collection('customer') do |collection|
  collection.replace_field_writing('job') do |value, _context|
    {
      'job' => value,
      'portfolio' => { 'title' => value }
    }
  end
end
```

#### DSL Syntax

Using simplified DSL with `collection`:

```ruby
@create_agent.collection :customer do |collection|
  collection.replace_writing :job do |value|
    {
      'job' => value,
      'portfolio' => { 'title' => value }
    }
  end
end
```

{% hint style="info" %}
If the relationships do not exist it will create them with the given field values.
{% endhint %}

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

#### Traditional Syntax

Using full context methods with `customize_collection`:

```ruby
@create_agent.customize_collection('customer') do |collection|
  collection.replace_field_writing('job') do |value, _context|
    {
      'job' => value,
      'portfolioId' => 8,
      'portfolio' => { 'title' => value }
    }
  end
end
```

#### DSL Syntax

Using simplified DSL with `collection`:

```ruby
@create_agent.collection :customer do |collection|
  collection.replace_writing :job do |value|
    {
      'job' => value,
      'portfolioId' => 8,
      'portfolio' => { 'title' => value }
    }
  end
end
```

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.

#### Traditional Syntax

Using full context methods with `customize_collection`:

```ruby
@create_agent.customize_collection('customer') do |collection|
  collection.replace_field_writing('job') do |value, _context|
    {
      'job' => value,
      'portfolioId' => 8,
      'portfolio' => {
        'title' => value,
        'format' => { 'name' => 'pdf' }
      }
    }
  end
end
```

#### DSL Syntax

Using simplified DSL with `collection`:

```ruby
@create_agent.collection :customer do |collection|
  collection.replace_writing :job do |value|
    {
      'job' => value,
      'portfolioId' => 8,
      'portfolio' => {
        'title' => value,
        'format' => { 'name' => 'pdf' }
      }
    }
  end
end
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.forestadmin.com/developer-guide-agents-ruby/agent-customization/fields/write.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.