How to seed a Postgres database with Knex

Diesel is my ORM & query builder of choice for Rust projects, it doesn’t provide a way to seed a database though. I went looking for a seeding crate and came up empty handed, so settled on looking for a Node package instead. Though even NPM had a surprisingly limited number options, I found Knex which has served me well so far.

Though we’ll just be using Knex’s seeding functionality in this post, it too is a fully functional query builder. Though I haven’t used it for query building, it’s syntax looks pretty nice and I would certainly consider it for any future Node projects.

Let’s get cracking then. I’m going to assume that you already have Postgres setup on your machine.

Setting up Knex

Once you’ve cd’d into your project, run the following to install the Knex package and the Node Postgres client needed by Knex:

$ yarn add knex pg --dev

Once that’s done, run the snippet below to create a new knexfile.js.

Note: If you prefer to use knex as a command, install it globally too with yarn global add knex then run knex init instead of this…

$ ./node_modules/.bin/knex init

Configuring knex

If you open the knexfile.js initialised above, you’ll see it houses the settings for development, staging & production databases. We’re just going to configure a test database in this post — from there, you’ll be able to configure any other environments you need.

Replace the contents of module.exports with the following configuration for our test db:

test: {
    client: 'pg',
    connection: 'postgres://username@localhost/your_app_name_test',
    migrations: {
        directory: __dirname + '/migrations'
    seeds: {
        directory: __dirname + '/seeds/test'

client, as the name suggests, specifies the database client we’re using for the environment. This post centres around PostgreSQL, Knex supports 6 different kinds of database though, including MySQL, MariaDB and Oracle.

connection is simply our database URL, which’ll need updating to the actual DB you’re targeting.

migrationsdirectory, as you might have guessed, is the path to the dir that houses your migrations. The above path is Diesel’s default — you’ll want to update it depending on where yours are kept.

seedsdirectory is where Knex will add generated seed files.

For a full list of configuration options, read through the relevant section of Knex’s documentation.

Creating seeds

With installation and configuration out of the way, we’re ready to start building some seeds. Back in your terminal, run the following to create a users seed file:

$ ./node_modules/.bin/knex seed:make users_seed --env test

That will generate a users_seed.js file and place it in the seeds directory we specified for our test environment earlier — ./seeds/test/.

Open up the file and you’ll see that it creates a nice little starting point for us. Though I prefer to dropdb a database before seeding it for tests, you can leave the .del() function in there if you like.

Start by updating table_name on lines 4 and 7 to the name of the table you’re seeding — in this case, users. You can then delete lines 8 through 10 and insert your own data. Note that in the generated file, the id of the seeded rows is being specified. Unless you know what you’re doing, don’t include the id in your own data as it will mess up BIGSERIAL auto-incrementation. An example entry for a user could look something like this:

return knex('users').insert([
        email: "[email protected]",
        password_hash: "$argon2d$m=4096,t=3,p=1$JGFyZ29uMmQkb[...]",
        password_hash_random_salt: "9FT2UFEWRZOFtz5CQ0542HRTHREs5lalk",
        password_reset_active: false,
        password_reset_hash: null,
        password_reset_time: null,
        first_name: "Elliot",
        last_name: "Jackson",
        created_at: "2017-04-20 12:29:45.964056",
        updated_at: "2017-04-20 12:29:45.964056",

Remember that we’re working with Node and that insert just takes an array of objects. That structure means that, depending on your needs, you can create large amounts of seed data very easily with something like faker and a simple loop that pushes to an array that is then passed to insert.


With all that said and done, we’re ready to actually seed our database which is no harder than running:

$ ./node_modules/.bin/knex seed:run

Wrapping up

Now that you know how to create and populate a single seed file for a single DB, you can start adding more data, more seed files and more environments by repeating the same methods outlined above.

The full documentation for Knex can be found here: