# To multiple records

{% hint style="success" %}
This is the official documentation of the `forestadmin-agent-django` and `forestadmin-agent-flask` Python agents.
{% endhint %}

Relationships that point to multiple records are displayed in the frontend in the "Related Data" and "Explorer" Tab.

![Explorer Tab](https://2921382565-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F2HgnlEINLUAEQC1KgN48%2Fuploads%2Fgit-blob-1bd805c9135a71be79e0bc41d15b82de74b76d93%2Frelationships-multiple-explorer.png?alt=media)

### One-to-Many relations

In a one-to-many relationship, one record from a Collection is attached to multiple records of another Collection.

Think about countries and towns: a country has multiple towns, and each town belongs to a country.

```python
# Link 'countries' to 'towns'
agent.customize_collection('countries').add_one_to_many_relation(
    name='myTowns',
    foreign_collections='towns',
    origin_key='country_id',
    origin_key_target='id', # Optional (uses primary key of countries by default)
)
```

### Many-to-Many relations

In a many-to-many relation, 3 Collections are used instead of 2 to build the relation.

This allows multiple records from one Collection to be attached to multiple records from another Collection.

For instance, on a movie recommendation website, each user can rate many movies, and each movie can be rated by many users. The 3 Collections that are used are `users` (the "origin" Collection), `ratings` (the "through" Collection), and `movies` (the "foreign" Collection).

```python
# Create one side of the relation ..
agent.customize_collection('users').add_many_to_many_relation(
    name='ratedMovies',
    foreign_collection='Movie',
    through_collection='Rating'
    origin_key='user_id',
    origin_key_target='id', # Optional (uses primary key of User by default)
    foreign_key='movie_id',
    foreign_key_target='id' # Optional (uses primary key of Movie by default)
)
```

### External relations

External relations allow to define Collections which will only be available through the "Related Data" section or a given model.

{% hint style="warning" %}
Note that external relations do not support pagination.
{% endhint %}

```python
from forestadmin.datasource_toolkit.context.collection_context import (
    CollectionCustomizationContext
)
from forestadmin.datasource_toolkit.interfaces.records import RecordsDataAlias

STATES = [
    {"code": "AK", "name": "Alaska", "zip": [99501, 99950], "closeTo": []},
    {
        "code": "AL",
        "name": "Alabama",
        "zip": [35004, 36925],
        "closeTo": ["TE", "MI", "GE"],
    },
    {
        "code": "AR",
        "name": "Arkansas",
        "zip": [71601, 72959],
        "closeTo": ["OK", "TX", "LO"],
    },
    {
        "code": "AZ",
        "name": "Arizona",
        "zip": [85001, 86556],
        "closeTo": ["NM", "CO", "NE"],
    },
    {
        "code": "CA",
        "name": "California",
        "zip": [90001, 96162],
        "closeTo": ["OR", "NE"]
    },
    # ....
]

async def near_states_list_records(
    record: RecordsDataAlias, context: CollectionCustomizationContext
) -> Any:
    ret = []
    if record["country"] == "USA":
        zip_code = record["zipCode"]
        for state in STATES:
            if not (state["zip"][0] < zip_code and zip_code < state["zip"][1]):
                continue
            ret.extend(filter(lambda s: s["code"] in state["closeTo"], STATES))

    return ret

# Create one side of the relation ..
agent.customize_collection('address').add_external_relation(
    "nearStates",
    {
        # Define schema of the records in the relation.
        "schema": { "code": "Sting", "name": "String" },

        # Which fields are needed from the parent record to run the handler?
        # Dependencies are optional: by default only the primary key of address
        # would be provided.
        "dependencies": ['country', 'zipCode'],

        # Compute list of records from the parent record
        "list_records": near_states_list_records
    }
)
```