A few weeks ago I started developing web applications with Node.JS and Express. This framework if you can call it that and very disconcerting at first, especially for a developer like me who has been studying with Ruby on Rails. But after several weeks of use, I realized that Node.JS is Express are wonderful.
However, I find that despite the popularity of Node.JS it is still difficult to find examples of structured applications. Thanks to the great flexibility of Node.JS we find lots of ways to do it but very few of them have a clean and maintainable code.
The biggest difficulty I have encountered is the management of database relationships. While sequelize presents itself as an essential ORM of Node.JS, the documentation and resources that can be found often leave much to be desired. That’s why after spending a lot of time managing this relationship, I decided to write this article in order to detail the implementation of all types of relationships possible thanks to Sequelize
The 3 types of relationship
We will take as an example 3 models.
- Company
- Employee
- Working day
Employe has one company (belongsTo) 1:1
Company has many employees (hasMany) 1:n
Employe has many WorkingDay and WorkingDay has many employees (manyToMany) n:n
To create many to many relationship, we will need a joining table that we will call WorkingDaysEmploye.
Express Generator and Sequelize CLI
I know that some people criticize the express generator or the sequelize cli. Personally I find that it allows both of us to start a project by giving a very suitable starting structure.
If not already done, globally install express-generator and sequelize-cli like this
npm install express-generator -g
npm install -g sequelize-cliThen we will generate a new app thanks to express and install the necessary libraries
express relationship-app-demo --no-view
cd relationship-app-demo
npm install --save sequelize mysql2
sequelize initThe last command will create 3 folders (Models, Migrations and Seeder) and a `config.json` file in the `/config` directory
We need to modify this file with your credentials to the local mysql server like this
after configuration you can run
`sequelize db:create` to create a new database named `nodejs_relationship_demo`
Models generation
now, we’re going to create all the models we’re going to need
sequelize model:generate --name Company --attributes name:STRINGsequelize model:generate --name User --attributes email:STRING,firstName:STRING,lastName:STRING,companyId:INTEGERsequelize model:generate --name WorkingDay --attributes weekDay:STRING,workingDate:DATE,isWorking:BOOLEANsequelize model:generate --name UsersWorkingDay --attributes userId:INTEGER,workingDayId:INTEGER
UsersWorkingDay will be used to join User and WorkingDay (many to many relationship).
Now that we have generated the necessary templates, we will have to indicate the relationships in the migration files by adding a reference key on all foreign keys.
I also added for each foreign key an AllowNull false to prevent creation if the model is not linked
You can then check with any database explorer that the tables are linked to each other.
The relationships being well defined on the mysql side, we will now have to explain to sequelize what the relationships are between our models
Models associations
1:1
The first relationship is a User belongs to a Company, open the `models/user.js` and add the association line 10 like this :
1:N
Knowing that a company has several Users we can add the 1:n relationship in the Company model. I rename the user model using an alias (employes) for more clarity, I put this alias in the plural because the relationship is hasMany
N:N
Finally, there is only one relationship left to manage and probably the most difficult. For this, we will need to describe the association in the 3 files concerned. (user.js, workingday.js, usersworkingday.js)
And that’s it, we configured the database and the sequelize models with the 3 types of relationships. It may seem simple after all, but before that, long hours of research were needed.
How to test relationships
The last thing to do once the configuration is finished, is to test the relationships between them to make sure that everything works as you want it to.
For that, I create a new test file that I will delete later
touch relationshipTester.jsFirst, import all the models we need
The first registration we will need is Company, so let’s start by creating a new company.
Now, we will create 3 Users (employees) linked to this company
Let’s see now if we can recover a company from an User and Users from a Company
Everything here is perfect! All that remains is to test the many to many relationship in this way.
Let’s start by creating 3 working days
Finally, we want to be able to retrieve the list of employees for a given work day, but also to know the work days of a given employee.
Conclusion
First I would like to apologize for the horror mistake that is in the code, I realized when writing this article that in English employee to take two ‘e’. (sorry, I’m French)
I would then say that what we need to remember is that when associating in models, it is a good practice to use aliases (as:) because sequelize pluralizes the name of models and it can sometimes bring surprises, by using aliases, you rename the associations which then allows you to use this alias in include. Sequelize also create functions for each association, we also use aliases in singular or plural to call these functions. The list is available here in the documentation.
Finally, during an belongsToMany association it is not mandatory but specifying the foreign key ensures a good connection. It also happened to me that Sequelize does not associate the model with the right table if it happens to you, you can specify a classname in the second parameter of the model definition.
Eth3rnit3
