# Models with Relationships in FastAPI
If we go right now and read a single **hero** by ID, we get the hero data with the team ID.
But we don't get any data about the particular team:
We get a response of:
```JSON hl_lines="5"
{
"name": "Deadpond",
"secret_name": "Dive Wilson",
"age": null,
"team_id": 1,
"id": 1,
}
```
And the same way, if we get a **team** by ID, we get the team data, but we don't get any information about this team's heroes:
Here we get a response of:
```JSON
{
"name": "Preventers",
"headquarters": "Sharp Tower",
"id": 2
}
```
...but no information about the heroes.
Let's update that. π€
## Why Aren't We Getting More Data
First, why is it that we are not getting the related data for each hero and for each team?
It's because we declared the `HeroRead` with only the same base fields of the `HeroBase` plus the `id`. But it doesn't include a field `team` for the **relationship attribute**.
And the same way, we declared the `TeamRead` with only the same base fields of the `TeamBase` plus the `id`. But it doesn't include a field `heroes` for the **relationship attribute**.
```Python hl_lines="3-5 9-10 14-19 23-24"
# Code above omitted π
{!./docs_src/tutorial/fastapi/teams/tutorial001.py[ln:7-9]!}
# Code here omitted π
{!./docs_src/tutorial/fastapi/teams/tutorial001.py[ln:22-23]!}
# Code here omitted π
{!./docs_src/tutorial/fastapi/teams/tutorial001.py[ln:32-37]!}
# Code here omitted π
{!./docs_src/tutorial/fastapi/teams/tutorial001.py[ln:46-47]!}
# Code below omitted π
```
π Full file preview
```Python
{!./docs_src/tutorial/fastapi/teams/tutorial001.py!}
```
Now, remember that FastAPI uses the `response_model` to validate and **filter** the response data?
In this case, we used `response_model=TeamRead` and `response_model=HeroRead`, so FastAPI will use them to filter the response data, even if we return a **table model** that includes **relationship attributes**:
```Python hl_lines="3 8 12 17"
# Code above omitted π
{!./docs_src/tutorial/fastapi/teams/tutorial001.py[ln:105-110]!}
# Code here omitted π
{!./docs_src/tutorial/fastapi/teams/tutorial001.py[ln:160-165]!}
# Code below omitted π
```
π Full file preview
```Python
{!./docs_src/tutorial/fastapi/teams/tutorial001.py!}
```
## Don't Include All the Data
Now let's stop for a second and think about it.
We cannot simply include *all* the data, including all the internal relationships, because each **hero** has an attribute `team` with their team, and then that **team** also has an attribute `heroes` with all the **heroes** in the team, including this one.
If we tried to include everything, we could make the server application **crash** trying to extract **infinite data**, going through the same hero and team over and over again internally, something like this:
```JSON hl_lines="2 13 24 34"
{
"name": "Rusty-Man",
"secret_name": "Tommy Sharp",
"age": 48,
"team_id": 1,
"id": 1,
"team": {
"name": "Preventers",
"headquarters": "Sharp Tower",
"id": 2,
"heroes": [
{
"name": "Rusty-Man",
"secret_name": "Tommy Sharp",
"age": 48,
"team_id": 1,
"id": 1,
"team": {
"name": "Preventers",
"headquarters": "Sharp Tower",
"id": 2,
"heroes": [
{
"name": "Rusty-Man",
"secret_name": "Tommy Sharp",
"age": 48,
"team_id": 1,
"id": 1,
"team": {
"name": "Preventers",
"headquarters": "Sharp Tower",
"id": 2,
"heroes": [
...with infinite data here... π±
]
}
}
]
}
}
]
}
}
```
As you can see, in this example, we would get the hero **Rusty-Man**, and from this hero we would get the team **Preventers**, and then from this team we would get its heroes, of course, including **Rusty-Man**... π±
So we start again, and in the end, the server would just crash trying to get all the data with a `"Maximum recursion error"`, we would not even get a response like the one above.
So, we need to carefully choose in which cases we want to include data and in which not.
## What Data to Include
This is a decision that will depend on **each application**.
In our case, let's say that if we get a **list of heroes**, we don't want to also include each of their teams in each one.
And if we get a **list of teams**, we don't want to get a list of the heroes for each one.
But if we get a **single hero**, we want to include the team data (without the team's heroes).
And if we get a **single team**, we want to include the list of heroes (without each hero's team).
Let's add a couple more **data models** that declare that data so we can use them in those two specific *path operations*.
## Models with Relationships
Let's add the models `HeroReadWithTeam` and `TeamReadWithHeroes`.
We'll add them **after** the other models so that we can easily reference the previous models.
```Python hl_lines="3-4 7-8"
# Code above omitted π
{!./docs_src/tutorial/fastapi/relationships/tutorial001.py[ln:61-66]!}
# Code below omitted π
```
π Full file preview
```Python
{!./docs_src/tutorial/fastapi/relationships/tutorial001.py!}
```
These two models are very **simple in code**, but there's a lot happening here. Let's check it out.
### Inheritance and Type Annotations
The `HeroReadWithTeam` **inherits** from `HeroRead`, which means that it will have the **normal fields for reading**, including the required `id` that was declared in `HeroRead`.
And then it adds the **new field** `team`, which could be `None`, and is declared with the type `TeamRead` with the base fields for reading a team.
Then we do the same for the `TeamReadWithHeroes`, it **inherits** from `TeamRead`, and declares the **new field** `heroes`, which is a list of `HeroRead`.
### Data Models Without Relationship Attributes
Now, notice that these new fields `team` and `heroes` are not declared with `Relationship()`, because these are not **table models**, they cannot have **relationship attributes** with the magic access to get that data from the database.
Instead, here these are only **data models** that will tell FastAPI **which attributes** to get data from and **which data** to get from them.
### Reference to Other Models
Also, notice that the field `team` is not declared with this new `TeamReadWithHeroes`, because that would again create that infinite recursion of data. Instead, we declare it with the normal `TeamRead` model.
And the same for `TeamReadWithHeroes`, the model used for the new field `heroes` uses `HeroRead` to get only each hero's data.
This also means that, even though we have these two new models, **we still need the previous ones**, `HeroRead` and `TeamRead`, because we need to reference them here (and we are also using them in the rest of the *path operations*).
## Update the Path Operations
Now we can update the *path operations* to use the new models.
This will tell **FastAPI** to take the object that we return from the *path operation function* (a **table model**) and **access the additional attributes** from them to extract their data.
In the case of the hero, this tells FastAPI to extract the `team` too. And in the case of the team, to extract the list of `heroes` too.
```Python hl_lines="3 8 12 17"
# Code above omitted π
{!./docs_src/tutorial/fastapi/relationships/tutorial001.py[ln:113-118]!}
# Code here omitted π
{!./docs_src/tutorial/fastapi/relationships/tutorial001.py[ln:168-173]!}
# Code below omitted π
```
π Full file preview
```Python
{!./docs_src/tutorial/fastapi/relationships/tutorial001.py!}
```
## Check It Out in the Docs UI
Now let's try it out again in the **docs UI**.
Let's try again with the same **hero** with ID `1`:
Now we get the **team** data included:
```JSON hl_lines="7-11"
{
"name": "Deadpond",
"secret_name": "Dive Wilson",
"age": null,
"team_id": 1,
"id": 1,
"team": {
"name": "Z-Force",
"headquarters": "Sister Margaretβs Bar",
"id": 1
}
}
```
And if we get now the **team** with ID `2`:
Now we get the list of **heroes** included:
```JSON hl_lines="5-41"
{
"name": "Preventers",
"headquarters": "Sharp Tower",
"id": 2,
"heroes": [
{
"name": "Rusty-Man",
"secret_name": "Tommy Sharp",
"age": 48,
"team_id": 2,
"id": 2
},
{
"name": "Spider-Boy",
"secret_name": "Pedro Parqueador",
"age": null,
"team_id": 2,
"id": 3
},
{
"name": "Tarantula",
"secret_name": "Natalia Roman-on",
"age": 32,
"team_id": 2,
"id": 6
},
{
"name": "Dr. Weird",
"secret_name": "Steve Weird",
"age": 36,
"team_id": 2,
"id": 7
},
{
"name": "Captain North America",
"secret_name": "Esteban Rogelios",
"age": 93,
"team_id": 2,
"id": 8
}
]
}
```
## Recap
Using the same techniques to declare additional **data models**, we can tell FastAPI what data to return in the responses, even when we return **table models**.
Here we almost **didn't have to change the FastAPI app** code, but of course, there will be cases where you need to get the data and process it in different ways in the *path operation function* before returning it.
But even in those cases, you will be able to define the **data models** to use in `response_model` to tell FastAPI how to validate and filter the data.
By this point, you already have a very robust API to handle data in a SQL database combining **SQLModel** with **FastAPI**, and implementing **best practices**, like data validation, conversion, filtering, and documentation. β¨
In the next chapter, I'll tell you how to implement automated **testing** for your application using FastAPI and SQLModel. β