# 3. Engine Setup

## 3.1 Engine Creation

The `Engine` constructor `__init__` establishes the core MongoDB connection with centralized naming configuration. It
accepts:

- `db`: MongoDB database connection (Motor/AgnosticDatabase)
- `collection_name_format`: Callable to generate collection names from model classes (default: class name)
- `link_name_format`: Callable to generate field names for document relationships (default: field alias)

These format parameters allow consistent naming rules across all bound documents.

Example of engine creation:

```
motor = AsyncIOMotorClient("localhost")
engine = Engine(motor["butty_test"], link_name_format=lambda f: f.alias + "_id")
```

## 3.2 Document Binding

`bind()` registers document classes with the engine, processing:

- Collection name resolution (via `DocumentConfig` or format callable)
- Field injection and relationship pipeline setup
- Duplicate binding prevention

The automatic document registry (populated via `__init_subclass__`) simplifies binding in complex applications by
tracking document classes. When `bind()` is called without parameters, it uses all registered documents. The
registration is carefully filtered to exclude:

- Classes with `registry=False` explicit override
- Abstract base classes (ABC)
- Generic type definitions (containing '\[')
- Pydantic internal models (`pydantic.main`)

This filtering is necessary because `__init_subclass__` triggers during various Pydantic setup operations, not just
actual document class definitions. The registry system provides convenience while preventing unwanted automatic binding
of support classes.

`unbind()` removes all document associations, clearing both explicitly bound documents and registry-tracked classes
while maintaining the underlying collections.

Example of document binding:

```python
engine.bind()  # binds all registered documents
engine.bind(User, Department)  # binds only User and Department
```

## 3.3 Engine Initialization

The `init()` method finalizes engine setup by creating all configured database indexes.

Example of engine initialization:

```python
async def main():
    await engine.init()
```