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201 lines
4.9 KiB
Markdown
201 lines
4.9 KiB
Markdown
# Create Models with a Many-to-Many Link
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We'll now support **many-to-many** relationships using a **link table** like this:
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<img alt="many-to-many table relationships" src="/img/tutorial/many-to-many/many-to-many.svg">
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Let's start by defining the class models, including the **link table** model.
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## Link Table Model
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As we want to support a **many-to-many** relationship, now we need a **link table** to connect them.
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We can create it just as any other **SQLModel**:
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```Python hl_lines="6-12"
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{!./docs_src/tutorial/many_to_many/tutorial001.py[ln:1-12]!}
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# Code below omitted 👇
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```
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<details>
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<summary>👀 Full file preview</summary>
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```Python
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{!./docs_src/tutorial/many_to_many/tutorial001.py!}
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```
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</details>
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This is a **SQLModel** class model table like any other.
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It has two fields, `team_id` and `hero_id`.
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They are both **foreign keys** to their respective tables. We'll create those models in a second, but you already know how that works.
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And **both fields are primary keys**. We hadn't used this before. 🤓
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## Team Model
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Let's see the `Team` model, it's almost identical as before, but with a little change:
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```Python hl_lines="8"
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# Code above omitted 👆
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{!./docs_src/tutorial/many_to_many/tutorial001.py[ln:15-20]!}
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# Code below omitted 👇
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```
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<details>
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<summary>👀 Full file preview</summary>
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```Python
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{!./docs_src/tutorial/many_to_many/tutorial001.py!}
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```
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</details>
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The **relationship attribute `heroes`** is still a list of heroes, annotated as `List["Hero"]`. Again, we use `"Hero"` in quotes because we haven't declared that class yet by this point in the code (but as you know, editors and **SQLModel** understand that).
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We use the same **`Relationship()`** function.
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We use **`back_populates="teams"`**. Before we referenced an attribute `team`, but as now we can have many, we'll rename it to `teams` when creating the `Hero` model.
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And here's the important part to allow the **many-to-many** relationship, we use **`link_model=HeroTeamLink`**. That's it. ✨
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## Hero Model
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Let's see the other side, here's the `Hero` model:
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```Python hl_lines="9"
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# Code above omitted 👆
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{!./docs_src/tutorial/many_to_many/tutorial001.py[ln:23-29]!}
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# Code below omitted 👇
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```
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<details>
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<summary>👀 Full file preview</summary>
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```Python
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{!./docs_src/tutorial/many_to_many/tutorial001.py!}
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```
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</details>
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We **removed** the previous `team_id` field (column) because now the relationship is done via the link table. 🔥
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The relationship attribute is now named **`teams`** instead of `team`, as now we support multiple teams.
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It is no longer an `Optional[Team]` but a list of teams, annotated as **`List[Team]`**.
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We are using the **`Relationship()`** here too.
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We still have **`back_populates="heroes"`** as before.
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And now we have a **`link_model=HeroTeamLink`**. ✨
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## Create the Tables
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The same as before, we will have the rest of the code to create the **engine**, and a function to create all the tables `create_db_and_tables()`.
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```Python hl_lines="9"
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# Code above omitted 👆
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{!./docs_src/tutorial/many_to_many/tutorial001.py[ln:32-39]!}
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# Code below omitted 👇
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```
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<details>
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<summary>👀 Full file preview</summary>
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```Python
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{!./docs_src/tutorial/many_to_many/tutorial001.py!}
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```
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</details>
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And as in previous examples, we will add that function to a function `main()`, and we will call that `main()` function in the main block:
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```Python hl_lines="4"
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# Code above omitted 👆
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{!./docs_src/tutorial/many_to_many/tutorial001.py[ln:78-79]!}
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# We will do more stuff here later 👈
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{!./docs_src/tutorial/many_to_many/tutorial001.py[ln:83-84]!}
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```
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<details>
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<summary>👀 Full file preview</summary>
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```Python
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{!./docs_src/tutorial/many_to_many/tutorial001.py!}
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```
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</details>
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## Run the Code
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If you run the code in the command line, it would output:
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<div class="termy">
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```console
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$ python app.py
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// Boilerplate omitted 😉
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INFO Engine
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CREATE TABLE team (
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id INTEGER,
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name VARCHAR NOT NULL,
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headquarters VARCHAR NOT NULL,
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PRIMARY KEY (id)
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)
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INFO Engine [no key 0.00033s] ()
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INFO Engine
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CREATE TABLE hero (
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id INTEGER,
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name VARCHAR NOT NULL,
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secret_name VARCHAR NOT NULL,
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age INTEGER,
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PRIMARY KEY (id)
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)
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INFO Engine [no key 0.00016s] ()
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INFO Engine
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// Our shinny new link table ✨
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CREATE TABLE heroteamlink (
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team_id INTEGER,
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hero_id INTEGER,
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PRIMARY KEY (team_id, hero_id),
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FOREIGN KEY(team_id) REFERENCES team (id),
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FOREIGN KEY(hero_id) REFERENCES hero (id)
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)
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INFO Engine [no key 0.00031s] ()
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INFO Engine COMMIT
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```
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</div>
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## Recap
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We can support **many-to-many** relationships between tables by declaring a link table.
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We can create it the same way as with other **SQLModel** classes, and then use it in the `link_model` parameter to `Relationship()`.
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Now let's work with data using these models in the next chapters. 🤓
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