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In this video I want to take a moment to introduce a really really important concept called associations

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so associations will allow us to have multiple pieces of data multiple collections in our database that

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are related to one another.

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And this is hugely important to making more complex web apps.

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Right now we have Yelp camp and your camp has campgrounds in it.

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But then we're going to add in users.

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Want to be able to have users who then are associated with campgrounds and then campgrounds will have

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comments and we want to have those campgrounds associated with a list of comments or if we take an example

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like Facebook on Facebook you sign up.

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So there's an entity a model called user and then there's another entity called post and then somewhere

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there's photos but there's also albums and there's comments and there's tags and likes.

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There are way more models that I'm not mentioning but all of these are interconnected.

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So a user is related to a post when I create a post.

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That post is not just an arbitrary post.

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It's related to me.

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Same thing with the photos that I upload and the albums that those photos belong to.

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So a photo can be associated with an album and an album can be associated with a user.

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And we have comments.

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A comment can be on a photo the comment can be on a post.

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I think a comment can also be on an album of photos than we have tags so photos can have tags but you

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can also tag people and comments.

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You can tag people in posts we have likes.

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You can like comment.

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You can like an album you can like a photo you can like a post.

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There is so many different things going on but the core concept is that data is related.

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The term associations just refers to this idea of having associated data and the other important point

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is that there are different types of associations there are associations that are known as one to one

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one to many and many to many relationships.

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So start by talking about one to one relationships and one to one are the simplest relationships.

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So we have one of an entity that's related to one another entity so we could say that one book has one

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publisher or an employee has one title where it gets more exciting as we start talking about one to

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many relationships and one to many relationship one entity.

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It's related to many of another entity.

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We can take the example of Facebook one user can have multiple photos that they have uploaded the photos

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belong to that user but those photos belong to one user.

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A photo can not have multiple uploads so it's a one to many relationship unlike a many to many relationship

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where the association goes both ways.

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The classic example of a many to many relationship is students and courses.

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So in college a student can sign up for multiple courses and each course has multiple students enrolled

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.

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So its this two way many to many relationship.

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Another one would be books and authors.

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So an author can write can have many books and each book can also have many authors.

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It might be more common for there to only be one author but it's definitely possible and not uncommon

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to have multiple authors.

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So those are the three basic types of relationships and the type that will encounter the most is one

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too many.

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So we'll have comments and campgrounds and one campground has many comments but one comment only belongs

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to one campground in the next two videos.

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I'm going to introduce how we can write associations how we can connect different models using mongoose

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and I'm going to show two different ways of doing that.

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The first is using what's known as embedding data and the second one it's known as referencing data

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.

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So I'm going to do these in two separate installments.

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And coming up next I'm going to show you how we can embed data to Associated