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Associations and Relationships
Database

Associations and Relationships

Associations are the Pop way to define a relation between two objects in the database. In this chapter, you’ll learn how to define associations using struct tags; and how to manipulate them with the Eager() modifier.

Example

type User struct {
  ID           uuid.UUID
  Email        string
  Password     string
  Books        Books     `has_many:"books" order_by:"title asc"`
  FavoriteSong Song      `has_one:"song" fk_id:"u_id"`
  Houses       Addresses `many_to_many:"users_addresses"`
}

type Book struct {
  ID      uuid.UUID
  Title   string
  Isbn    string
  User    User        `belongs_to:"user"`
  UserID  uuid.UUID
}

type Song struct {
  ID      uuid.UUID
  Title   string
  UserID  uuid.UUID   `db:"u_id"`
}

type Address struct {
  ID           uuid.UUID
  Street       string
  HouseNumber  int
}

type Books []Book
type Addresses []Address

Available Struct Tags

Using the above example code below is a list of available struct tags and how to use them.

  • has_many: This tag is used to describe one-to-many relationships in the database. In the example, User type defines a one-to-many relation with Books slice type through the use of has_many tag, meaning a User can own many Books. When querying to the database, Pop will load all records from the books table that have a column named user_id, or the column specified with fk_id that matches the User.ID value.

  • belongs_to: This tag is used to describe the owner in the relationship. An owner represents a highly coupled dependency between the model and the target association field where belongs_to tag was defined. This tag is mostly used to indicate that model owns its “existence” to the association field with belongs_to. In the example above, Book type use belongs_to to indicate that it is owned by a User type. When querying to the database, Pop will load a record from the users table with id that matches with Book.UserID value.

  • has_one: This tag is used to describe one-to-one relationships in the database. In the example above, there is only one FavoriteSong within all songs records that User type like the most. When querying to the database, Pop will load a record from the songs table that have a column named user_id, or the column specified with fk_id that matches the User.ID field value.

  • many_to_many: This tag is used to describe many-to-many relationships in the database. In the example above, the relationship between User type and Addresses slice type exists to indicate an User can own many Houses and a House can be owned by many Users. It is important to notice that value for many_to_many tag is the associative table that connects both sides in the relationship; in the example above this value is defined as users_addresses. When querying to the database, Pop will load all records from the addresses table through the associative table users_addresses. Table users_addresses MUST define address_id and user_id columns to match User.ID and Address.ID field values. You can also define a fk_id tag that will be used in the target association i.e. addresses table.

  • fk_id: This tag can be used to define the column name in the target association that matches model ID. In the example above, Song has a column named u_id that references the id of the users table. When loading FavoriteSong, u_id column will be used instead of user_id.

  • order_by: This tag can be used in combination with has_many and many_to_many tags to indicate the order for the association when loading. The format to use is order_by:"<column_name> <asc | desc>"

Loading Associations

Pop currently provides two modes for loading associations; each mode will affect the way pop loads associations and queries to the database.

Eager. Default mode. By enabling this mode, pop will perform “n” queries for every association defined in the model. This means more hits to the database in order to not affect memory use.

EagerPreload. Optional mode. By enabling this mode, pop will perform one query for every association defined in the model. This mode will hit the database with a reduced frequency by sacrifing more memory space.

  • pop.SetEagerMode: Pop allows enabling any of these modes globally which will affect ALL queries handle performance. Use EagerDefault or EagerPreload as parameter to activate any of these modes.

  • tx.EagerPreload | q.EagerPreload: Pop allows developers to take control in which situations they want Pop to perform any of these modes when necessary. This method will activate EagerPreload mode only for the query in action.

  • tx.Eager | q.Eager: Pop allows developers to take control in which situations they want Pop to perform any of these modes when necessary. This method will activate Eager mode only for the query in action.

Eager Mode

The pop.Connection.Eager() method tells Pop to load the associations for a model once that model is loaded from the database. This mode will perform “n” queries for every association defined in the model.

for i := 0; i < 3; i++ {
  user := User{ID: i + 1}
  tx.Create(&user)
}

for i := 0; i < 3; i++ {
  book := Book{UserID: i +1}
  tx.Create(&book)
}

u := Users{}
err := tx.Eager().All(&u)  // loads all associations for every user registered, i.e Books, Houses and FavoriteSong

Eager mode will:

  1. Load all users.
 SELECT * FROM users;
  1. Iterate on every user and load its associations:
 SELECT * FROM books WHERE user_id=1)

 SELECT * FROM books WHERE user_id=2)

 SELECT * FROM books WHERE user_id=3)

EagerPreload Mode

The pop.Connection.EagerPreload() method tells Pop to load the associations for a model once that model is loaded from the database. This mode will hit the database with a reduced frequency by sacrifing more memory space.

for i := 0; i < 3; i++ {
  user := User{ID: i + 1}
  tx.Create(&user)
}

for i := 0; i < 3; i++ {
  book := Book{UserID: i +1}
  tx.Create(&book)
}

u := Users{}
err := tx.EagerPreload().All(&u)  // loads all associations for every user registered, i.e Books, Houses and FavoriteSong

EagerPreload mode will:

  1. Load all users.
 SELECT * FROM users;
  1. Load associations for all users in one single query.
  SELECT * FROM books WHERE user_id IN (1,2,3))

Load Specific Associations

By default Eager and EagerPreload will load all the assigned associations for the model. To specify which associations should be loaded you can pass in the names of those fields to the Eager or EagerPreload methods and only those associations will be loaded.

err  = tx.Eager("Books").Where("name = 'Mark'").All(&u) // load only Books association for user with name 'Mark'.
// OR
err  = tx.EagerPreload("Books").Where("name = 'Mark'").All(&u) // load only Books association for user with name 'Mark'.

Pop also allows you to eager load nested associations by using the . character to concatenate them. Take a look at the example below.

// will load all Books for u and for every Book will load the user which will be the same as u.
tx.Eager("Books.User").First(&u)
// OR
tx.EagerPreload("Books.User").First(&u)

// will load all Books for u and for every Book will load all Writers and for every writer will load the Book association.
tx.Eager("Books.Writers.Book").First(&u)
// OR
tx.EagerPreload("Books.Writers.Book").First(&u)

// will load all Books for u and for every Book will load all Writers. And Also it will load the favorite song for user.
tx.Eager("Books.Writers").Eager("FavoriteSong").First(&u)
// OR
tx.EagerPreload("Books.Writers").EagerPreload("FavoriteSong").First(&u)

Loading Associations for an Existing Model

The pop.Connection.Load() method takes a model struct, that has already been populated from the database, and an optional list of associations to load.

tx.Load(&u) // load all associations for user, i.e Books, Houses and FavoriteSong
tx.Load(&u, "Books") // load only the Books associations for user

The Load method will not retrieve the User from the database, only its associations.

Flat Nested Creation

Pop allows you to create the models and their associations with other models in one step by default. You no longer need to create every association separately anymore. Pop will even create join table records for many_to_many associations.

Assuming the following pieces of pseudo-code:

book := Book{Title: "Pop Book", Description: "Pop Book", Isbn: "PB1"}
tx.Create(&book)
song := Song{Title: "Don't know the title"}
tx.Create(&song)
addr := Address{HouseNumber: 1, Street: "Golang"}
tx.Create(&addr)

user := User{
  Name: "Mark Bates",
  Books: Books{Book{ID: book.ID}},
  FavoriteSong: song,
  Houses: Addresses{
    addr,
  },
}

err := tx.Create(&user)

  1. It will notice Books is a has_many association and it will realize that to actually update each book it will need to get the User ID first. So, it proceeds to store first User data so it can retrieve an ID and then use that ID to fill UserID field in every Book in Books. It updates all affected books in the database using their IDs to target them.

  2. FavoriteSong is a has_one association and it uses same logic described in has_many association. Since User data was previously saved before updating all affected books, it already knows that User has got an ID so it fills its UserID field with that value and FavoriteSong is then updated in the database.

  3. Houses in this example is a many_to_many relationship and it will have to deal with two tables in this case: users and addresses. Because User was already stored, it already has its ID. It will then use the IDs passed with the Addresses to create the coresponding entries in the join table.

For a belongs_to association like shown in the example below, it fills its UserID field before being saved in the database.

book := Book{
   Title:      "Pop Book",
   Description: "Pop Book",
   Isbn:        "PB1",
   User: user,
}

tx.Create(&book)

Eager Creation

Pop also allows you to create models and embed the creation of their associations in one step as well.

Assuming the following pieces of pseudo-code:

user := User{
  Name: "Mark Bates",
  Books: Books{{Title: "Pop Book", Description: "Pop Book", Isbn: "PB1"}},
  FavoriteSong: Song{Title: "Don't know the title"},
  Houses: Addresses{
    Address{HouseNumber: 1, Street: "Golang"},
  },
}

err := tx.Eager().Create(&user)

  1. It will notice Books is a has_many association and it will realize that to actually store every book it will need to get the User ID first. So, it proceeds to first store/create the User data so it can retrieve an ID and then use that ID to fill the UserID field in every Book in Books. Later it stores all books in the database.

  2. FavoriteSong is a has_one association and it uses same logic described in the has_many association. Since User data was previously saved before creating all books, it already knows that User has got an ID so it fills its UserID field with that value and FavoriteSong is then stored in the database.

  3. Houses in this example is a many_to_many relationship and it will have to deal with two tables, in this case: users and addresses. It will need to store all addresses first in the addresses table before saving them in the many to many(join) table. Because User was already stored, it already has an ID. * This is a special case to deal with, since this behavior is different from all other associations, it is solved by implementing the AssociationCreatableStatement interface, all other associations by default implement the AssociationCreatable interface.

For a belongs_to association like shown in the example below, it will need to first create the User to retrieve its ID value and then fill its UserID field before being saved in the database.

book := Book{
   Title:      "Pop Book",
   Description: "Pop Book",
   Isbn:        "PB1",
   User: User{
        Name: nulls.NewString("Larry"),
   },
}

tx.Eager().Create(&book)

In the case where you feed the eager create with associated models that already exist, it will, instead of creating duplicates of them or updating the contents of them, simply create/update the associations with them.

Next Steps