# Extra Models { #extra-models }

Continuing with the previous example, it will be common to have more than one related model.

This is especially the case for user models, because:

* The **input model** needs to be able to have a password.
* The **output model** should not have a password.
* The **database model** would probably need to have a hashed password.

/// danger

		t.Error(err)
	}

	lvl := &Level3{}
	if err := DB.Save(lvl).Error; err != nil {
		t.Error(err)
	}

	sublvl1 := &Level4{Level3ID: lvl.ID}
	if err := DB.Save(sublvl1).Error; err != nil {
		t.Error(err)
	}
	sublvl2 := &Level4{Level3ID: lvl.ID}
	if err := DB.Save(sublvl2).Error; err != nil {
		t.Error(err)
	}

	lvl.Level4s = []*Level4{sublvl1, sublvl2}

Notice that the default value is still `None`, so the parameter is still optional.

But now, having `Query(max_length=50)` inside of `Annotated`, we are telling FastAPI that we want it to have **additional validation** for this value, we want it to have maximum 50 characters. 😎

/// tip

// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

package cmd

import (
	"context"
	"encoding/binary"
	"errors"
	"fmt"

Let's say you have a file structure as described in [Bigger Applications](bigger-applications.md){.internal-link target=_blank}:

```
.
├── app
│   ├── __init__.py
│   └── main.py
```

In the file `main.py` you have your **FastAPI** app:


{* ../../docs_src/app_testing/main.py *}

### Testing file { #testing-file }

{* ../../docs_src/dependencies/tutorial008_an_py39.py hl[18:19,26:27] *}

The same way, you could have some dependencies with `yield` and some other dependencies with `return`, and have some of those depend on some of the others.

And you could have a single dependency that requires several other dependencies with `yield`, etc.

You can have any combinations of dependencies that you want.

 *
 *   protected Scheduler scheduler() {
 *     return Scheduler.newFixedRateSchedule(0, 1, TimeUnit.SECONDS);
 *   }
 * }
 * }
 *
 * <p>This class uses the life cycle methods to read in a list of starting URIs and save the set of
 * outstanding URIs when shutting down. Also, it takes advantage of the scheduling functionality to
 * rate limit the number of queries we perform.
 *
 * @author Luke Sandberg
 * @since 11.0
 */

  /*
   * Ideally, this class would have exposed only constructors that require a non-null cause. See
   * https://github.com/jspecify/jspecify-reference-checker/blob/61aafa4ae52594830cfc2d61c8b113009dbdb045/src/main/java/com/google/jspecify/nullness/NullSpecTransfer.java#L789
   * and https://github.com/jspecify/jspecify/issues/490.
   *
   * (Perhaps it should also have required that its cause was a RuntimeException. However, that

That's why the current versions are still `0.x.x`, this reflects that each version could potentially have breaking changes. This follows the <a href="https://semver.org/" class="external-link" target="_blank">Semantic Versioning</a> conventions.

You can create production applications with **FastAPI** right now (and you have probably been doing it for some time), you just have to make sure that you use a version that works correctly with the rest of your code.

        assertEquals(1, stats.getRdmaReads(), "Should have 1 read operation");
        assertEquals(1, stats.getRdmaWrites(), "Should have 1 write operation");
        assertEquals(1, stats.getRdmaSends(), "Should have 1 send operation");
        assertEquals(1, stats.getRdmaReceives(), "Should have 1 receive operation");
        assertEquals(5120, stats.getBytesTransferred(), "Should have transferred 5KB total");