Override writing behavior

This is the official documentation of the forestadmin-agent-django and forestadmin-agent-flask Python agents.

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 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.

Making a field read-only

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

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.

from forestadmin.datasource_toolkit.decorators.write.write_replace.write_customization_context import (
    WriteCustomizationContext,
)
from forestadmin.datasource_toolkit.interfaces.records import RecordsDataAlias

def full_name_write_fn(
    value, context: WriteCustomizationContext
) -> RecordsDataAlias:
    first_name, last_name = value.split(' ')
    return {"firstName": first_name, "LastName": last_name}

collection.replace_field_writing('fullName', full_name_write_fn)

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.

from datetime import date
from forestadmin.datasource_toolkit.decorators.write.write_replace.write_customization_context import (
    WriteCustomizationContext,
)
from forestadmin.datasource_toolkit.interfaces.records import RecordsDataAlias

def first_name_write_fn(
    value, context: WriteCustomizationContext
) -> RecordsDataAlias:
    if context.action == "create":
        return {"firstName": value, "firstNameLastEdited": None}
    elif context.action == "update":
        return {"firstName": value, "firstNameLastEdited": date.today().isoformat()}
    else:
      raise Exception("Unexpected value")

collection.replace_field_writing('firstName', first_name_write_fn)

Changing fields in related records

Handling relationships inside a replace_field_writing 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.replace_field_writing(
    "job", lambda value, context: {"job": value, "portfolio": {"title": value}}
)

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.replace_field_writing(
    "job",
    lambda value, context: {
        "job": value,
        "portfolioId": 8,
        "portfolio": {"title": value},
    },
)

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.replace_field_writing(
    "job",
    lambda value, context: {
        "job": value,
        "portfolioId": 8,
        "portfolio": {"title": value, "format": {"name": "pdf"}},
    },
)