Collection hook

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

Forest Admin allows customizing at a very low level the behavior of any given Collection via the usage of Collection Hooks.

Collection Hooks are a very powerful feature and require special care when using them.

How it works

Any given Collection should implement all of the following functions:

• list

• create

• update

• delete

• aggregate

The Collection Hooks feature allows executing code before and/or after any of these functions, providing an easy way to interact with your Collections.

To declare a Hook on a Collection, the following information is required:

• A lifecycle position (Before | After)

• An action type (List | Create | Update | Delete | Aggregate)

• A callback, that will receive a context matching the provided hook position and hook definition.

Context object reference

All hook contexts provide access to:

• collection - the current collection, which can be queried using the Forest Admin Query Interface

• dataSource - the composite data source containing all collections

• caller - information about the user performing the operation

The caller object

The caller object contains information about the current user:

Property	Description
id	User ID
email	User email
firstName	First name
lastName	Last name
team	Team name
role	Role name
tags	Custom tags
timezone	User timezone

Error methods

All contexts provide methods to throw errors that will be displayed in the Forest Admin UI:

Method	Description
throw_validation_error(message)	Display a validation error
throw_forbidden_error(message)	Display a forbidden error
throw_error(message)	Display a generic error

Hook-specific context properties

Each hook type provides additional properties:

Hook	Position	Properties
List	Before	filter (paginated filter), projection (fields to return)
List	After	Same as Before + records (returned records)
Create	Before	data (data to create)
Create	After	data + records (created records)
Update	Before	filter, patch (read-only)
Update	After	filter, patch (read-only)
Delete	Before/After	filter (filter for records to delete)
Aggregate	Before	filter, aggregation, limit
Aggregate	After	Same as Before + aggregateResult

A single Collection can have multiple Hooks with the same position and the same type. They will run in their declaration order. Collection Hooks are only called when the Collection function is contacted by the UI. This means that any usage of the Forest Admin query interface will not trigger them.

Basic use cases

In the following example, we want to prevent a set of users from updating any records of the Transactions table. We want to check if the user email is allowed to update a record via an external API call.

from forestadmin.datasource_toolkit.decorators.hook.context.update import (
    HookBeforeUpdateContext
)

async def transaction_before_update_fn(context: HookBeforeUpdateContext):
    is_allowed = my_function_to_check_user_is_allowed(context.caller.email)
    if not is_allowed:
        context.throw_forbidden_error(f"{context.caller.email} is not allowed")

agent.customize_collection("Transactions").add_hook(
    "Before", "Update", transaction_before_update_fn
)

Another good example would be the following: Each time a new User is created in the database, I want to send him an email.

agent.customize_collection("User").add_hook(
    "After",
    "Create",
    lambda context: MyEmailSender.send_email(
        {
            "from": "[email protected]",
            "to": context.records[0]['email'],
            "message": "Hey, a new account was created with this email.",
        }
    ),
)

Advanced examples

Preventing deletion of protected records

async def before_delete_user(context):
    records = await context.collection.list(context.filter, ['protected', 'email'])

    protected_records = [r for r in records if r.get('protected')]
    if protected_records:
        emails = ', '.join(r['email'] for r in protected_records)
        context.throw_forbidden_error(f"Cannot delete protected users: {emails}")

agent.customize_collection("user").add_hook("Before", "Delete", before_delete_user)

Querying other collections via dataSource

async def before_create_order(context):
    customers_collection = context.datasource.get_collection('customer')

    for order in context.data:
        filter = ConditionTreeLeaf('id', 'equal', order['customer_id'])
        customers = await customers_collection.list(filter, ['credit_limit', 'current_balance'])
        customer = customers[0]

        if customer['current_balance'] + order['amount'] > customer['credit_limit']:
            context.throw_validation_error(f"Order exceeds credit limit for customer {order['customer_id']}")

agent.customize_collection("order").add_hook("Before", "Create", before_create_order)