API with NestJS #75. Many-to-many relationships using raw SQL queries

JavaScript NestJS SQL

This entry is part 75 of 77 in the API with NestJS

Designing relationships between tables is one of the crucial parts of working with databases. In this article, we look into a more complex relationship called many-to-many.

You can find the code from this article in this repository.

The many-to-many relationship

A many-to-many relationship happens when many records in one table relate to many records in another table. A good example is a connection between posts and categories. A particular post can be published under multiple categories. For example, this article falls under both the SQL and JavaScript categories. On the other hand, a single category can be related to numerous different posts.

So far, we’ve worked with one-to-one or many-to-one relationships using raw SQL queries. In the above approaches, we use a simple column containing a foreign key that matches a row from a different table.

The case gets complicated when we want to create a connection between one post and multiple categories. We shouldn’t put multiple values in the column. To implement a many-to-many relationship, we create a joining table.

Creating the table allows us to store the relationships between particular categories and posts.

Creating the many-to-many relationship

Let’s define a migration that creates the and tables.


An important thing to notice in how we created the table is that it doesn’t have a separate column. Instead, we specify a composite primary key. This approach has some advantages. First, we save a bit of disk space thanks to not creating the column. But more importantly, we make sure it is unique thanks to marking a combination of the and as the primary key. All rows in a table should have a different primary key. Thanks to that, the following data would never appear in our table:

By the above, we ensure that a particular post might relate to a particular category only once.

Connecting posts to categories

When a user publishes a post, it can be related to multiple categories. For example, we might accept the following data through our API:

The above means that we want to add two rows to the table:

Fortunately, we can insert multiple rows into a table simultaneously. One way of doing that is inserting a result of a   query:

To understand the above code, we need to take a closer look at this query:

Above, we use the function to expand an array to a set of rows. Thanks to that, our query returns multiple rows that the statement saves into the database.

We can now use all of the above knowledge to create a post and connect it to categories in the same query.


We also need to create a model that includes the property.


Thanks to the above, we can now create posts and connect them to categories in a single query.

Fetching the ids of categories of a certain post

So far, when fetching the details of a certain post, we’ve attached the details of an author. Let’s take it a step further, and attach the ids of the categories related to the post. Let’s break down this problem into a simple set of steps to perform.

First, we need to get all the rows from the table related to a particular post.

We can parse it into a single array to make it easier to work with.

Let’s prepare a new model to handle the above data.


We now have everything we need to fetch a post with its author and category ids.


Fetching all posts from a certain category

There is a big chance that we will want to get a list of all the posts from a certain category. To achieve this, we need to join the data from the table with .

Let’s break down this problem into smaller chunks. First, we must fetch all post ids from a certain category.

Since we know the ids of all the posts, we can use the   statement to match them with the rows from the table.

Let’s create a new model to prepare for the above data.


We now can use all of the above to:

  • fetch the data of a particular category,
  • match it with related posts,
  • fit the data into the model.

We can use the above logic to fetch the details of the category when it is requested.


In this article, we’ve gone through the many-to-many relationship. When doing that, we implemented an example with posts and categories. To do that, we learned how to manage a joining table and insert multiple entities into the database with one query. There is still more to learn about using NestJS with raw SQL queries, so stay tuned!

Series Navigation<< API with NestJS #74. Designing many-to-one relationships using raw SQL queriesAPI with NestJS #76. Working with transactions using raw SQL queries >>
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