sqlmodel/docs/tutorial/connect/create-connected-tables.md

Create Connected Tables

Now we will deal with connected data put in different tables.

So, the first step is to create more than one table and connect them, so that each row in one table can reference another row in the other table.

We have been working with heroes in a single table hero. Let's now add a table team.

The team table will look like this:

id	name	headquarters
1	Preventers	Sharp Tower
2	Z-Force	Sister Margaret’s Bar

To connect them, we will add another column to the hero table to point to each team by the ID with the team_id:

id	name	secret_name	age	team_id ✨
1	Deadpond	Dive Wilson	null	2 ✨
2	Spider-Boy	Pedro Parqueador	null	1 ✨
3	Rusty-Man	Tommy Sharp	48	1 ✨

This way each row in the table hero can point to a row in the table team:

One-to-Many and Many-to-One

Here we are creating connected data in a relationship where one team could have many heroes. So it is commonly called a one-to-many or many-to-one relationship.

The many-to-one part can be seen if we start from the heroes, many heroes could be part of one team.

This is probably the most popular type of relationship, so we'll start with that. But there's also many-to-many and one-to-one relationships.

Create Tables in Code

Create the team Table

Let's start by creating the tables in code.

Import the things we need from sqlmodel and create a new Team model:

{!./docs_src/tutorial/connect/create_tables/tutorial001.py[ln:1-9]!}

# Code below omitted 👇

👀 Full file preview

{!./docs_src/tutorial/connect/create_tables/tutorial001.py!}

This is very similar to what we have been doing with the Hero model.

The Team model will be in a table automatically named "team", and it will have the columns:

• id, the primary key, automatically generated by the database
• name, the name of the team
• headquarters, the headquarters of the team

And finally we mark it as a table in the config.

Create the New hero Table

Now let's create the hero table.

This is the same model we have been using up to now, we are just adding the new column team_id:

{!./docs_src/tutorial/connect/create_tables/tutorial001.py[ln:1-18]!}

# Code below omitted 👇

👀 Full file preview

{!./docs_src/tutorial/connect/create_tables/tutorial001.py!}

Most of that should look familiar:

The column will be named team_id. It will be an integer, and it could be NULL in the database (or None in Python), becase there could be some heroes that don't belong to any team.

As we don't have to explicitly pass team_id=None when creating a hero, we add a default of None to the Field().

Now, here's the new part:

In Field() we pass the argument foreign_key="team.id". This tells the database that this column team_id is a foreign key to the table team. A "foreign key" just means that this column will have the key to identify a row in a foreign table.

The value in this column team_id will be the same integer that is in some row in the id column on the team table. That is what connects the two tables.

The Value of foreign_key

Notice that the foreign_key is a string.

Inside it has the name of the table, then a dot, and then the name of the column.

This is the name of the table in the database, so it is "team", not the name of the model class Team (with a capital T).

If you had a custom table name, you would use that custom table name.

!!! info You can learn about setting a custom table name for a model in the Advanced User Guide.

Create the Tables

Now we can add the same code as before to create the engine and the function to create the tables:

# Code above omitted 👆

{!./docs_src/tutorial/connect/create_tables/tutorial001.py[ln:21-28]!}

👀 Full file preview

{!./docs_src/tutorial/connect/create_tables/tutorial001.py!}

And as before, we'll call this function from another function main(), and we'll add that function main() to the main block of the file:

# Code above omitted 👆

{!./docs_src/tutorial/connect/create_tables/tutorial001.py[ln:31-36]!}

👀 Full file preview

{!./docs_src/tutorial/connect/create_tables/tutorial001.py!}

Run the Code

!!! tip Before running the code, make sure you delete the file database.db to make sure you start from scratch.

If we run the code we have up to now, it will go and create the database file database.db and the tables in it we just defined, team and hero:

$ python app.py

// Automatically start a new transaction
INFO Engine BEGIN (implicit)

// Check if the tables exist already
INFO Engine PRAGMA main.table_info("team")
INFO Engine [raw sql] ()
INFO Engine PRAGMA temp.table_info("team")
INFO Engine [raw sql] ()
INFO Engine PRAGMA main.table_info("hero")
INFO Engine [raw sql] ()
INFO Engine PRAGMA temp.table_info("hero")
INFO Engine [raw sql] ()

// Create the tables
INFO Engine 
CREATE TABLE team (
        id INTEGER, 
        name VARCHAR NOT NULL, 
        headquarters VARCHAR NOT NULL, 
        PRIMARY KEY (id)
)


INFO Engine [no key 0.00010s] ()
INFO Engine 
CREATE TABLE hero (
        id INTEGER, 
        name VARCHAR NOT NULL, 
        secret_name VARCHAR NOT NULL, 
        age INTEGER, 
        team_id INTEGER, 
        PRIMARY KEY (id), 
        FOREIGN KEY(team_id) REFERENCES team (id)
)


INFO Engine [no key 0.00026s] ()
INFO Engine COMMIT

Create Tables in SQL

Let's see that same generated SQL code.

As we saw before, those VARCHAR columns are converted to TEXT in SQLite, which is the database we are using for these experiments.

So, the first SQL could also be written as:

CREATE TABLE team (
    id INTEGER, 
    name TEXT NOT NULL, 
    headquarters TEXT NOT NULL, 
    PRIMARY KEY (id)
)

And the second table could be written as:

CREATE TABLE hero (
    id INTEGER, 
    name TEXT NOT NULL, 
    secret_name TEXT NOT NULL, 
    age INTEGER, 
    team_id INTEGER, 
    PRIMARY KEY (id), 
    FOREIGN KEY(team_id) REFERENCES team (id)
)

The only new is the FOREIGN KEY line, and as you can see, it tells the database what column in this table is a foreign key (team_id), which other (foreign) table it references (team) and which column in that table is the key to define which row to connect (id).

Feel free to experiment with it in DB Browser for SQLite.

Recap

Using SQLModel, in most of the cases you only need a field (column) with a foreign_key in the Field() with a string pointing to another table and column to connect two tables.

Now that we have the tables created and connected, let's create some rows in the next chapter. 🚀