# Beego DB primary key and Relationship Explore

## One to One

Means a relationship between to structs where one instance of one struct can be related to one instance of the other struct.
Suppose, there is a Facebook user. The user can have only one profile as that user. So if User and Profile are two models then their relationship is one-to-one.

type User struct {
    UserId int
    Name string
    Age int
    Profile  *Profile `orm:"rel(one)"`
}

type Profile struct {
    Id int // primary key
    Education string
    Workplace string
    ContactNumber string
    User *User `orm:"reverse(one)"`  // foreign key
}

## One to many
Example, a facebook user can have multiple posts. If user and post are two structs then user has one to many relationship with post. To integret this with orm following steps can be performed.

type User struct {
    UserId int
    Name string
    Age int
    Posts  []*Post `orm:"reverse(many)"`
}

type Post struct {
    Id int // primary key
    Heading string
    Topic string
    User *User `orm:"rel(fk)"`  // foreign key
}

## Many to many
Again coming to facebook, a post can have multiple tags and same tag can be usd in multiple posts. In database world this relationship is called many to many.

type Tag struct {
    Id int
    Name string
    Posts  []*Post `orm:"reverse(many)"`
}

type Post struct {
    Id int // primary key
    Heading string
    Topic string
    Tags []*Tag `orm:"rel(m2m)"`  // foreign key
}

**Could not verify many to many relationship using*

## Setting Primary key

By default when a table is created, Beego creates an Id field as primary key. However, we can set a primary key of our choice as well while declaring the struct for the model.

*/models/user.go*

type User struct {
    Id int
    Name string
Age int
}

When the model is created for above struct, Beego Orm takes `Id` as primary key.

Changed */models/user.go*

type User struct {
    Id int
    UserId string `orm:"pk"`
    Name string
    Age int
}

// demo function to generate the UserId as it cannot be incremental here
func generateUserId(Age int, name string) string {
    return strconv.ItoA(Age)+"/"+name
}

### Links
1. [Key setting](https://blog.csdn.net/Charles_Thanks/article/details/80502829)
2. [Beego Documentation](https://beego.me/docs/mvc/model/models.md)
3. [Primary Key Value related](https://ddcode.net/2019/04/14/beegos-orm-gives-the-primary-key-value-every-updatedeleteread/)
4. [Primary key field using Beego](https://developpaper.com/question/encounter-need-a-primary-key-field-when-using-beego/)