What is Association?
Association will allow us to have multiple pieces of data collections in our database that are related to one another. Just like a Facebook user can have multiple posts or images in his account.
Three Types of Association
One to One:
One of entity that’s related to one another entity.For example, one student has one unique student ID.
One to Many:
One entity that’s related to many of other entity. For example, one student has multiple social media accounts. And those social media accounts only belong to this student.
Many to Many:
Many entity related to both ways. For example, one student can take many course, and one course has many students.
Embedded Data
In this example, we will do the “One to Many” type. We have user and post. We want one user to have multiple posts.
Step One: Set up and define schema
Firstly, we need to create a new file “embed,js”, and npm install mongoose. Then, we require the mongoose and connect to MongoDB inside the file. And, we need to make sure the MongoDB is running in another window.
Then, we define user and post schemas by giving each of them an object with related items. For user, we need to have email, and name. For post, we need to have post title and content. And then, we put each schema into a mongoose model.
Step Two: Test New User
We create a new user with name “Mary” and email “mary@nyu.edu”. And then, we save the user data into the database. As we run the server, it shows the new user information with unique id. And we use the same way to create new post and test successful.
Step Three: Embed Data
In order to relate the user to post, we need to embed data. To do that,we need to have a post attribute inside the user. Inside the post attribute, we will have many posts. So, we need to add the post schema inside of an array. Therefore, we tell the mongoose we want it to be a list which is an array of posts.
We also need to move the post schema to the top, so Node can find the post variable first, then related to the user schema.
Step Four: Push post to a new user
We create another new user. Then, we push the posts to the new user which we defined: newUser.posts.push(). Then, we save the new user and new post.
As result, it shows the new user with new posts in the database. The post has an array which includes the post title and content.
Step Five: Retrieve the data
We can use user.findOne({}, function())to retrieve the a particular data from database. In this case, we retrieve Lily’s data, and we see all her information and post.
Step Six: Add new post
We can also push the post to the same user inside the retrieve function and save the data.
Once we run the server, it shows Lily has two posts in the post array.
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