# Collection override

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This is the official documentation of the `forestadmin-agent-django` and `forestadmin-agent-flask` Python agents.
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Forest Admin allows customizing at a very low level the behavior of any given Collection via the usage of Collection Overrides.

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Collection Overrides provide a powerful means to completely replace the default behavior of CUD operations (`create`, `update`, `delete`) for your Collections. This feature should be used with caution, as it directly affects the core operations on your data.
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### How it Works

In addition to the standard Collection functions:

* `create`
* `update`
* `delete`

Collection Overrides allow you to define custom behavior that will entirely replace the default implementation of the `create`, `update`, and `delete` operations.

To define an Override for a Collection, you must specify:

* The handler function that will be executed instead of the default operation.

The custom handler function will receive a context object containing relevant information for the operation, allowing for comprehensive control over the behavior of these CUD operations.

### Context object reference

All override contexts provide access to:

* `collection` - the current collection
* `datasource` - the composite data source containing all collections
* `caller` - information about the user performing the operation (see [Collection Hooks](https://docs.forestadmin.com/developer-guide-agents-python/agent-customization/collection-hook#the-caller-object) for details)

#### Override-specific context properties

| Override   | Properties                                                             |
| ---------- | ---------------------------------------------------------------------- |
| **Create** | `data` - Array of records to create                                    |
| **Update** | `filter` - Filter for records to update, `patch` - Partial update data |
| **Delete** | `filter` - Filter for records to delete                                |

### Setting Up Overrides

Overrides are declared similarly to hooks but are aimed at replacing an entire operation rather than augmenting its execution. However this can also be used to enrich the default behavior. Here's how to set up overrides in your Collection:

#### Custom Create Operation

To replace the default create operation, use `override_create` with your custom handler:

```python
from forestadmin.datasource_toolkit.decorators.override.context import CreateOverrideCustomizationContext

async def create_handle(context: CreateOverrideCustomizationContext)
    # Custom logic to handle creation
    # context.data contains the data intended for creation
    # Return an array of created records

collection.override_create(create_handle)
```

#### Custom Update Operation

To replace the default update operation, use `override_update` with your custom handler:

```python
from forestadmin.datasource_toolkit.decorators.override.context import UpdateOverrideCustomizationContext

async def update_handle(context: UpdateOverrideCustomizationContext)
    # Custom logic to handle update
    # context.filter to determine which records are targeted
    # context.patch contains the data for update
    # Perform update operation

collection.override_update(update_handle)
```

#### Custom Delete Operation

To replace the default delete operation, use `override_delete` with your custom handler:

```python
from forestadmin.datasource_toolkit.decorators.override.context import DeleteOverrideCustomizationContext

async def delete_handle(context: DeleteOverrideCustomizationContext)
    # Custom logic to handle deletion
    # context.filter to determine which records are targeted
    # Perform deletion operation

collection.override_delete(delete_handle)
```

{% hint style="info" %}
In theses examples, we use async function as handler, but you can also use sync or lambda function.
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{% hint style="warning" %}
Overrides take precedence over the default operation. Ensure your custom handlers properly manage all necessary logic for the operation, as the default behavior will not be executed.
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### Basic Use Cases

#### Create over API

You might want to create the record with your custom API:

```python
import requests
from forestadmin.datasource_toolkit.decorators.override.context import CreateOverrideCustomizationContext

def create_handle(context: CreateOverrideCustomizationContext)
    response = requests.post("https://my-product-api.com/products", json=context.data)
    return response.json()

collection.override_create(create_handle)
```

#### Modify data before update

You might want to modify payload data before update your record:

```python
import requests
from forestadmin.datasource_toolkit.decorators.override.context import UpdateOverrideCustomizationContext

async def update_handle(context: UpdateOverrideCustomizationContext)
    # Execute data modification and validation only if one of name or slug was edited
    if "name" in context.patch or "slug" in context.patch:
        name = context.get("name", context["slug"].split("-")[0])
        response = requests.get("https://my-product-api.com/slug", json={"name": name})
        uuid = response.text

        context.patch["name"] = name
        context.patch["slug"] = f"{name}-{uuid}"

    await context.collection.update(context.filter, context.patch)

collection.override_update(update_handle)
```

#### Implementing soft delete

Instead of actually deleting records, mark them as deleted:

```python
from forestadmin.datasource_toolkit.decorators.override.context import DeleteOverrideCustomizationContext

async def soft_delete_handle(context: DeleteOverrideCustomizationContext):
    # Get all records that would be deleted
    records = await context.collection.list(context.filter, ['id'])

    # Mark them as deleted instead of actually deleting
    for record in records:
        filter = ConditionTreeLeaf('id', 'equal', record['id'])
        await context.collection.update(filter, {
            'deleted_at': datetime.now(),
            'deleted_by': context.caller.id
        })

collection.override_delete(soft_delete_handle)
```