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In this video.

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Let's talk about how we can relate to or more document together.

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And the first hype is what we call embedding method, meaning we can relate to or more document together

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using this method called embedding method.

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And I will explain to you as we move on.

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The next hype is what you call referencing.

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So among these, which one should we use?

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Well, let's dive in more to talk about each of these type.

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Let's start with embedding method.

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So what is embedding way of relating document together?

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Let me show you this picture.

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Well, to understand what is called embedding in this case, let's say we are developing a blog application

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and we have two collections.

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One collection is called the user collection, and this is what you call the user schema.

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And you can see that for this user schema, we have these properties called Full Name Country and then

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post where I will come up to this and let's say that you also have two posts post one and then post

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two and each post it has a title and then ID call post one ID and then post to ID.

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So these two posts are from post collection and then this one is from where user collection.

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So it means that we have two collections.

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So how are we going to relate?

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Simply means that if I want to know how many posts are all post that has been created by a certain user,

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we need what is called relating document together and the first type is what you call the embedding.

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So what does it mean?

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Let's have a look at the user's collection or the user schema here.

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If you look at this field, we have post and in there we have arrays of post ID and the post ID one

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represent this post and then post number to represent this post to.

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In this case we are relating to document together.

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So it means that if I fetch user, I'm going to see all the posts that I have created and this is what

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we call the embedding, meaning that we are embedding the IDs into the fields of post of the user,

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and for embedding we can take the whole object and then place it inside the post array than the ID.

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We can use the entire object and place it inside as that, or we can use the IDs and if we want to fetch

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we are going to look through the collection and then convert all these IDs into normal posts.

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And this is what we call the embedding way.

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But guys, this option has some limitations and drawbacks.

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Let's take it that if I want to find or in my profile page, I don't want to fetch my post, but because

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we have embedded, it means that any time we make requests to the user or the post this user has made

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will be fetched.

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Let's take it that this user has created 1000 post.

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You can see how we are going to consume resources.

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And again, MongoDB has limitation on how many size of documents a particular field can take or a collection

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can take.

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Again, another limitation is that let's say that in future I have updated post number one to post number

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for a title.

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So I need to come to my collection of users and also update that post manually.

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Or I can use what is called middleware for Mongoose to do that work and wouldn't it be a little bit

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difficult?

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But in some cases you may need this kind of embedding in some use cases.

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Case number one is that if you know that this field isn't going to consume or contain a lot of documents,

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then this is the best way to go.

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And again, if you know that the document inside this array is not going to be updated frequently in

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the future, then this is the best way to go.

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So what is the next type of relating the command together?

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And that is what we call the reference end.

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This is the most recommended.

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All right.

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So what is referencing again, let's say we have the same collection from the previous video.

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We have user module or user schema, and then we have also the post schema.

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But before I explain, let's look at the files inside each schema.

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For the user, we have the full name called John Doe and then country called UAC.

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And then on the post we have a title and then the user who is creating this particular post.

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Well, it will say that inside we are using special syntax called mongoose dot schema, dot types,

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dot object ID and we are referencing the user, which is this.

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So when you are using referencing these two documents are independent, they are not related together.

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But because we use what is called mongoose schema dot hypes, dot object ID and the user meaning the

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reference we are using what is called the reference in way.

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Now here comes the question that I know you may be thinking of.

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Question number one is that how will I know which user created a particular post?

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And it's pretty simple one.

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You could see that we have the user here, meaning that when you are creating a post you need to provide

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the ID and this one means the mongoose ID, meaning the user ID and you provide it here.

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So right now we will know which user has created this particular post.

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Right now, next question is how will I know?

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How many posts have I created?

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Because there is no field or post on the user model.

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But behind the scene we call something virtual properties.

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When we say virtual properties, they are properties that are not on this schema.

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But when we execute a method, for example, find we can populate those virtual fields upon making a

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request like find or find or find by ID and you will see those fields in the next video.

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Let's start with how we can model a data where a user can create many posts and a post can belong to

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a single user.