> For the complete documentation index, see [llms.txt](https://softstack-factory.gitbook.io/mean-stack/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://softstack-factory.gitbook.io/mean-stack/backend/wk8-final-project-relations/relations.md).
# Relations
Relations allow us to have one model be related another model.
### Before you start
We will be altering how the **`Favorite`** model works with our backend in this gitbook notebook.
Originally we made the **`Favorite`** model without attaching it to an user of the angular application aka the **`AppUser`** model. We also added an entry to the favorites collection in our database to see if we could store something on your database.
{% hint style="warning" %}
We will need to delete any **`Favorite`** entries in our database before we implement the below steps of making the **`Favorite`** model be related to the **`AppUser`**Model.
{% endhint %}
We need to do this because we are altering the way these two models will work together, otherwise you'd have some inconsistent data in your database. Some **`Favorite`** model entries will be related to **`AppUser`** and some will not.
### **Summary/Overview**
{% hint style="info" %}
**Relations allow us to have one model be related another model.**
{% endhint %}
For example:
The illustration above represents a **`Customer`** model having potentially many **`Order`** model entries, this illustrates the **"**relation**"**.
[Example reference ](https://loopback.io/doc/en/lb3/HasMany-relations.html)
Another Example:
An **`AppUser`** can have many **`Favorites` .** This will allow us to retrieve information related to a user, more specifically what **`Favorite`** model entries a particular **AppUser** has stored in the database
{% tabs %}
{% tab title="AppUser Model Entry" %}
```typescript
appUser = {
id: "3afsdjk212gl3af"
firstName: "Jane",
lastName: "Doe",
email: "JaneDoe@mail.com"
}
```
In the above example the **id** from the **`AppUser`** model entry **(line 2)** is inserted as a “**foreign key**” on the **`Favorite`** model entry **(line 3)** on the **`Favorite`** *Model Entry Tab*.
{% endtab %}
{% tab title="Favorite Model Entry" %}
```typescript
favorite = {
id: "5dfac7c7f809983c88298247"
userId: "3afsdjk212gl3af"
movieTitle: "Star Wars",
thirdPartyMovieId: 11,
posterPath: "/btTdmkgIvOi0FFip1sPuZI2oQG6.jpg"
}
```
**Line 3: `Favorite`** entry's `userId` value "**3afsdjk212gl3af**" is from the **`appUser`** entry
{% endtab %}
{% endtabs %}
{% hint style="warning" %}
In loopback each instance will have a unique id generated for it automatically on created unless otherwise specified.
{% endhint %}
## **Relation Setup**
#### **Step 1**
In your terminal type: **`lb relation`**
#### **Step 2**
Now Select **`appUser`** as the model we are starting this relation from.
#### **Step 3**
Now we will select the type of relation. In our case an **`appUser`** can have many **favorites**. So we select hasMany
#### **Step 4**
Now we have the choice of the other model we want to have the relation with. In this case **`favorite`** is model that an **`appUser`** can have many of.
#### **Step 5**
Now we need a property name for the relation, favorites. This is property that will show up in our model that is the owner for example. In this case our **`appUser`** has many **`favorites`**.
The name in the parentheses( ) below is the default name of the relation unless you type in another name.
Press **`Enter`**
#### **Step 6**
Now we enter a foreign key this will be the field added to the **`favorite`** model. In the example below **`userId`** is the foreign key that refers to the id of the **`appUser`** who it is related to.
| favorite = {
id: "5dfac7c7f809983c88298247"
userId: "3afsdjk212gl3af"
movieTitle: "Star Wars",
thirdPartyMovieId: 11,
posterPath: "/btTdmkgIvOi0FFip1sPuZI2oQG6.jpg"
}
|
| ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
`UserId`above will be the foreign key.
**Step 7**\
\
Require a through model would be the case of a lets say we have a doctor and a patient they would both own the appointment. The appointment would be the through model.\
We will be selecting no.
## **Relation Recap**
We set up a “has many” relation from the **`appUser`** model to **`Favorites`** because an **`appUser`** can own many favorites . After creating this relation the **`appUser`** model will have a new property called `favorites`, and a foreign key or `userId` property will be added to the **`favorite`** model instance which will point to the unique **`id`** of the **`appUser`**.
## **Relations in Loopback Explorer**
Now restart your backend to update the new changes and go to the explorer.\
Select the **`appUser`** endpoint
Then scroll down and you should have new routes that will have do to with the created relation. In our case **favorites**. Below are a list of pre-built routes that take advantage of the relation we created.
After you setup the token authentication and access control settings, you can use the token stored in our sessionStorage object to make a http call in angular to submit a new **`Favorite`** model entry for a particular **`appUser`** to the database.
{% code title="main.component.ts" %}
```typescript
AddToFavorites(){
let token = sessionStorage.getItem("token")
let favoritesUrl = "http://localhost:3000/api/appUsers/${sessionStorage.getItem("userId")}/favorites/?access_token=${token}`"
this._http.get( favoritesUrl )
.subscribe(
res => console.log("res", res)
)
}
```
{% endcode %}