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Knex query builder in a REST API

cesareferrari profile image Cesare Ferrari ・3 min read

Using Knex to retrieve one record by id

We now know how to retrieve, or get, all the posts in a database table named posts with Knex (see previous article for details).
The syntax is very simple:

db('posts') // gets all the records in table 'posts'

We now want to retrieve one single post. We will retrieve the post through a GET request to our API.
The request URL will look like this:

localhost:4000/api/posts/1

where 1 is the id of the record we want.

GET request

We first need to set up a GET route handler to handle that route in our API. We use an Express router bound to the '/api/posts' endpoint:

// server.js

const PostRouter = require('./posts/post-router.js');
server.use('/api/posts', PostRouter);

Inside this route handler we use knex to make the database query. Remember, knex uses an object called db to make queries.

Knex query

If we had to find a record in a database table by a certain id using plain SQL, the query would look something like this:

SELECT * FROM posts WHERE id = 1;

We use the same concept with knex. Knex has a where() method, that takes a Javascript object as an argument, which specifies the conditions of the WHERE clause.
In our case, we only have one condition, the id field must match, so our WHERE clause could look like this:

where({id: id})

In cases like this, when an object property name matches the variable name, Javascript will let us write a shorter syntax:

where({id})

So, this is our WHERE clause. But where does the id come from? Right, it comes from the request parameters. We can extract the id in this way:

const { id } = req.params;

Putting it all together

Now that we have extracted the id from the request parameter and defined the WHERE clause, we can finish our route handler:

router.get('/:id', async (req, res) => {

  const { id } = req.params;

  try {
    const [post] = await db('posts').where({id});

    if (post) {
      res.status(200).json(post)
    } else {
      res.status(404).json({message: "Post not found"})
    }
  } catch (err) {
    res.status(500).json({message: "Error getting posts", error: err})
  }
});

Note that the database query inside the try block returns a collection with one element. We extract this element from the array by assigning it to a constant with this syntax:

const [post] = await db('posts').where({id});

Also note that we use an if statement to make sure the post is found. If the post is not found, perhaps because there is no record in the database with a matching id, we send back a status of 404 and an error message.
In the catch block we handle all other types of errors, like problems with the database, with the connection, etc.

If we send a request now, with a correct item id, it will return the item in a JSON object:

Return item in a JSON object

If we request a non-existing id, we will get a “not found” message, like we wanted:

Item not found

As you can see, finding one single record is a little more involved than getting all the records but not by much.


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