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Edges

Quick Summary#

Edges are the relations (or associations) of entities. For example, user's pets, or group's users.

er-group-users

In the example above, you can see 2 relations declared using edges. Let's go over them.

1. pets / owner edges; user's pets and pet's owner -

ent/schema/user.go

package schema
import (
"entgo.io/ent"
"entgo.io/ent/schema/edge"
)
// User schema.
type User struct {
ent.Schema
}
// Fields of the user.
func (User) Fields() []ent.Field {
return []ent.Field{
// ...
}
}
// Edges of the user.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("pets", Pet.Type),
}
}

ent/schema/pet.go

package schema
import (
"entgo.io/ent"
"entgo.io/ent/schema/edge"
)
// Pet holds the schema definition for the Pet entity.
type Pet struct {
ent.Schema
}
// Fields of the Pet.
func (Pet) Fields() []ent.Field {
return []ent.Field{
// ...
}
}
// Edges of the Pet.
func (Pet) Edges() []ent.Edge {
return []ent.Edge{
edge.From("owner", User.Type).
Ref("pets").
Unique(),
}
}

As you can see, a User entity can have many pets, but a Pet entity can have only one owner.
In relationship definition, the pets edge is a O2M (one-to-many) relationship, and the owner edge is a M2O (many-to-one) relationship.

The User schema owns the pets/owner relationship because it uses edge.To, and the Pet schema just has a back-reference to it, declared using edge.From with the Ref method.

The Ref method describes which edge of the User schema we're referencing because there can be multiple references from one schema to other.

The cardinality of the edge/relationship can be controlled using the Unique method, and it's explained more widely below.

2. users / groups edges; group's users and user's groups -

ent/schema/group.go

package schema
import (
"entgo.io/ent"
"entgo.io/ent/schema/edge"
)
// Group schema.
type Group struct {
ent.Schema
}
// Fields of the group.
func (Group) Fields() []ent.Field {
return []ent.Field{
// ...
}
}
// Edges of the group.
func (Group) Edges() []ent.Edge {
return []ent.Edge{
edge.To("users", User.Type),
}
}

ent/schema/user.go

package schema
import (
"entgo.io/ent"
"entgo.io/ent/schema/edge"
)
// User schema.
type User struct {
ent.Schema
}
// Fields of the user.
func (User) Fields() []ent.Field {
return []ent.Field{
// ...
}
}
// Edges of the user.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.From("groups", Group.Type).
Ref("users"),
// "pets" declared in the example above.
edge.To("pets", Pet.Type),
}
}

As you can see, a Group entity can have many users, and a User entity can have have many groups.
In relationship definition, the users edge is a M2M (many-to-many) relationship, and the groups edge is also a M2M (many-to-many) relationship.

To and From#

edge.To and edge.From are the 2 builders for creating edges/relations.

A schema that defines an edge using the edge.To builder owns the relation, unlike using the edge.From builder that gives only a back-reference for the relation (with a different name).

Let's go over a few examples that show how to define different relation types using edges.

Relationship#

O2O Two Types#

er-user-card

In this example, a user has only one credit-card, and a card has only one owner.

The User schema defines an edge.To card named card, and the Card schema defines a back-reference to this edge using edge.From named owner.

ent/schema/user.go

// Edges of the user.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("card", Card.Type).
Unique(),
}
}

ent/schema/card.go

// Edges of the Card.
func (Card) Edges() []ent.Edge {
return []ent.Edge{
edge.From("owner", User.Type).
Ref("card").
Unique().
// We add the "Required" method to the builder
// to make this edge required on entity creation.
// i.e. Card cannot be created without its owner.
Required(),
}
}

The API for interacting with these edges is as follows:

func Do(ctx context.Context, client *ent.Client) error {
a8m, err := client.User.
Create().
SetAge(30).
SetName("Mashraki").
Save(ctx)
if err != nil {
return fmt.Errorf("creating user: %w", err)
}
log.Println("user:", a8m)
card1, err := client.Card.
Create().
SetOwner(a8m).
SetNumber("1020").
SetExpired(time.Now().Add(time.Minute)).
Save(ctx)
if err != nil {
return fmt.Errorf("creating card: %w", err)
}
log.Println("card:", card1)
// Only returns the card of the user,
// and expects that there's only one.
card2, err := a8m.QueryCard().Only(ctx)
if err != nil {
return fmt.Errorf("querying card: %w", err)
}
log.Println("card:", card2)
// The Card entity is able to query its owner using
// its back-reference.
owner, err := card2.QueryOwner().Only(ctx)
if err != nil {
return fmt.Errorf("querying owner: %w", err)
}
log.Println("owner:", owner)
return nil
}

The full example exists in GitHub.

O2O Same Type#

er-linked-list

In this linked-list example, we have a recursive relation named next/prev. Each node in the list can have only one next node. If a node A points (using next) to node B, B can get its pointer using prev (the back-reference edge).

ent/schema/node.go

// Edges of the Node.
func (Node) Edges() []ent.Edge {
return []ent.Edge{
edge.To("next", Node.Type).
Unique().
From("prev").
Unique(),
}
}

As you can see, in cases of relations of the same type, you can declare the edge and its reference in the same builder.

func (Node) Edges() []ent.Edge {
return []ent.Edge{
+ edge.To("next", Node.Type).
+ Unique().
+ From("prev").
+ Unique(),
- edge.To("next", Node.Type).
- Unique(),
- edge.From("prev", Node.Type).
- Ref("next).
- Unique(),
}
}

The API for interacting with these edges is as follows:

func Do(ctx context.Context, client *ent.Client) error {
head, err := client.Node.
Create().
SetValue(1).
Save(ctx)
if err != nil {
return fmt.Errorf("creating the head: %w", err)
}
curr := head
// Generate the following linked-list: 1<->2<->3<->4<->5.
for i := 0; i < 4; i++ {
curr, err = client.Node.
Create().
SetValue(curr.Value + 1).
SetPrev(curr).
Save(ctx)
if err != nil {
return err
}
}
// Loop over the list and print it. `FirstX` panics if an error occur.
for curr = head; curr != nil; curr = curr.QueryNext().FirstX(ctx) {
fmt.Printf("%d ", curr.Value)
}
// Output: 1 2 3 4 5
// Make the linked-list circular:
// The tail of the list, has no "next".
tail, err := client.Node.
Query().
Where(node.Not(node.HasNext())).
Only(ctx)
if err != nil {
return fmt.Errorf("getting the tail of the list: %v", tail)
}
tail, err = tail.Update().SetNext(head).Save(ctx)
if err != nil {
return err
}
// Check that the change actually applied:
prev, err := head.QueryPrev().Only(ctx)
if err != nil {
return fmt.Errorf("getting head's prev: %w", err)
}
fmt.Printf("\n%v", prev.Value == tail.Value)
// Output: true
return nil
}

The full example exists in GitHub.

O2O Bidirectional#

er-user-spouse

In this user-spouse example, we have a symmetric O2O relation named spouse. Each user can have only one spouse. If user A sets its spouse (using spouse) to B, B can get its spouse using the spouse edge.

Note that there are no owner/inverse terms in cases of bidirectional edges.

ent/schema/user.go

// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("spouse", User.Type).
Unique(),
}
}

The API for interacting with this edge is as follows:

func Do(ctx context.Context, client *ent.Client) error {
a8m, err := client.User.
Create().
SetAge(30).
SetName("a8m").
Save(ctx)
if err != nil {
return fmt.Errorf("creating user: %w", err)
}
nati, err := client.User.
Create().
SetAge(28).
SetName("nati").
SetSpouse(a8m).
Save(ctx)
if err != nil {
return fmt.Errorf("creating user: %w", err)
}
// Query the spouse edge.
// Unlike `Only`, `OnlyX` panics if an error occurs.
spouse := nati.QuerySpouse().OnlyX(ctx)
fmt.Println(spouse.Name)
// Output: a8m
spouse = a8m.QuerySpouse().OnlyX(ctx)
fmt.Println(spouse.Name)
// Output: nati
// Query how many users have a spouse.
// Unlike `Count`, `CountX` panics if an error occurs.
count := client.User.
Query().
Where(user.HasSpouse()).
CountX(ctx)
fmt.Println(count)
// Output: 2
// Get the user, that has a spouse with name="a8m".
spouse = client.User.
Query().
Where(user.HasSpouseWith(user.Name("a8m"))).
OnlyX(ctx)
fmt.Println(spouse.Name)
// Output: nati
return nil
}

Note that, the foreign-key column can be configured and exposed as an entity field using the Edge Field option as follows:

// Fields of the User.
func (User) Fields() []ent.Field {
return []ent.Field{
field.Int("spouse_id").
Optional(),
}
}
// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("spouse", User.Type).
Unique().
Field("spouse_id"),
}
}

The full example exists in GitHub.

O2M Two Types#

er-user-pets

In this user-pets example, we have a O2M relation between user and its pets. Each user has many pets, and a pet has one owner. If user A adds a pet B using the pets edge, B can get its owner using the owner edge (the back-reference edge).

Note that this relation is also a M2O (many-to-one) from the point of view of the Pet schema.

ent/schema/user.go

// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("pets", Pet.Type),
}
}

ent/schema/pet.go

// Edges of the Pet.
func (Pet) Edges() []ent.Edge {
return []ent.Edge{
edge.From("owner", User.Type).
Ref("pets").
Unique(),
}
}

The API for interacting with these edges is as follows:

func Do(ctx context.Context, client *ent.Client) error {
// Create the 2 pets.
pedro, err := client.Pet.
Create().
SetName("pedro").
Save(ctx)
if err != nil {
return fmt.Errorf("creating pet: %w", err)
}
lola, err := client.Pet.
Create().
SetName("lola").
Save(ctx)
if err != nil {
return fmt.Errorf("creating pet: %w", err)
}
// Create the user, and add its pets on the creation.
a8m, err := client.User.
Create().
SetAge(30).
SetName("a8m").
AddPets(pedro, lola).
Save(ctx)
if err != nil {
return fmt.Errorf("creating user: %w", err)
}
fmt.Println("User created:", a8m)
// Output: User(id=1, age=30, name=a8m)
// Query the owner. Unlike `Only`, `OnlyX` panics if an error occurs.
owner := pedro.QueryOwner().OnlyX(ctx)
fmt.Println(owner.Name)
// Output: a8m
// Traverse the sub-graph. Unlike `Count`, `CountX` panics if an error occurs.
count := pedro.
QueryOwner(). // a8m
QueryPets(). // pedro, lola
CountX(ctx) // count
fmt.Println(count)
// Output: 2
return nil
}

Note that, the foreign-key column can be configured and exposed as an entity field using the Edge Field option as follows:

// Fields of the Pet.
func (Pet) Fields() []ent.Field {
return []ent.Field{
field.Int("owner_id").
Optional(),
}
}
// Edges of the Pet.
func (Pet) Edges() []ent.Edge {
return []ent.Edge{
edge.From("owner", User.Type).
Ref("pets").
Unique().
Field("owner_id"),
}
}

The full example exists in GitHub.

O2M Same Type#

er-tree

In this example, we have a recursive O2M relation between tree's nodes and their children (or their parent).
Each node in the tree has many children, and has one parent. If node A adds B to its children, B can get its owner using the owner edge.

ent/schema/node.go

// Edges of the Node.
func (Node) Edges() []ent.Edge {
return []ent.Edge{
edge.To("children", Node.Type).
From("parent").
Unique(),
}
}

As you can see, in cases of relations of the same type, you can declare the edge and its reference in the same builder.

func (Node) Edges() []ent.Edge {
return []ent.Edge{
+ edge.To("children", Node.Type).
+ From("parent").
+ Unique(),
- edge.To("children", Node.Type),
- edge.From("parent", Node.Type).
- Ref("children").
- Unique(),
}
}

The API for interacting with these edges is as follows:

func Do(ctx context.Context, client *ent.Client) error {
root, err := client.Node.
Create().
SetValue(2).
Save(ctx)
if err != nil {
return fmt.Errorf("creating the root: %w", err)
}
// Add additional nodes to the tree:
//
// 2
// / \
// 1 4
// / \
// 3 5
//
// Unlike `Save`, `SaveX` panics if an error occurs.
n1 := client.Node.
Create().
SetValue(1).
SetParent(root).
SaveX(ctx)
n4 := client.Node.
Create().
SetValue(4).
SetParent(root).
SaveX(ctx)
n3 := client.Node.
Create().
SetValue(3).
SetParent(n4).
SaveX(ctx)
n5 := client.Node.
Create().
SetValue(5).
SetParent(n4).
SaveX(ctx)
fmt.Println("Tree leafs", []int{n1.Value, n3.Value, n5.Value})
// Output: Tree leafs [1 3 5]
// Get all leafs (nodes without children).
// Unlike `Int`, `IntX` panics if an error occurs.
ints := client.Node.
Query(). // All nodes.
Where(node.Not(node.HasChildren())). // Only leafs.
Order(ent.Asc(node.FieldValue)). // Order by their `value` field.
GroupBy(node.FieldValue). // Extract only the `value` field.
IntsX(ctx)
fmt.Println(ints)
// Output: [1 3 5]
// Get orphan nodes (nodes without parent).
// Unlike `Only`, `OnlyX` panics if an error occurs.
orphan := client.Node.
Query().
Where(node.Not(node.HasParent())).
OnlyX(ctx)
fmt.Println(orphan)
// Output: Node(id=1, value=2)
return nil
}

Note that, the foreign-key column can be configured and exposed as an entity field using the Edge Field option as follows:

// Fields of the Node.
func (Node) Fields() []ent.Field {
return []ent.Field{
field.Int("parent_id").
Optional(),
}
}
// Edges of the Node.
func (Node) Edges() []ent.Edge {
return []ent.Edge{
edge.To("children", Node.Type).
From("parent").
Unique().
Field("parent_id"),
}
}

The full example exists in GitHub.

M2M Two Types#

er-user-groups

In this groups-users example, we have a M2M relation between groups and their users. Each group has many users, and each user can be joined to many groups.

ent/schema/group.go

// Edges of the Group.
func (Group) Edges() []ent.Edge {
return []ent.Edge{
edge.To("users", User.Type),
}
}

ent/schema/user.go

// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.From("groups", Group.Type).
Ref("users"),
}
}

The API for interacting with these edges is as follows:

func Do(ctx context.Context, client *ent.Client) error {
// Unlike `Save`, `SaveX` panics if an error occurs.
hub := client.Group.
Create().
SetName("GitHub").
SaveX(ctx)
lab := client.Group.
Create().
SetName("GitLab").
SaveX(ctx)
a8m := client.User.
Create().
SetAge(30).
SetName("a8m").
AddGroups(hub, lab).
SaveX(ctx)
nati := client.User.
Create().
SetAge(28).
SetName("nati").
AddGroups(hub).
SaveX(ctx)
// Query the edges.
groups, err := a8m.
QueryGroups().
All(ctx)
if err != nil {
return fmt.Errorf("querying a8m groups: %w", err)
}
fmt.Println(groups)
// Output: [Group(id=1, name=GitHub) Group(id=2, name=GitLab)]
groups, err = nati.
QueryGroups().
All(ctx)
if err != nil {
return fmt.Errorf("querying nati groups: %w", err)
}
fmt.Println(groups)
// Output: [Group(id=1, name=GitHub)]
// Traverse the graph.
users, err := a8m.
QueryGroups(). // [hub, lab]
Where(group.Not(group.HasUsersWith(user.Name("nati")))). // [lab]
QueryUsers(). // [a8m]
QueryGroups(). // [hub, lab]
QueryUsers(). // [a8m, nati]
All(ctx)
if err != nil {
return fmt.Errorf("traversing the graph: %w", err)
}
fmt.Println(users)
// Output: [User(id=1, age=30, name=a8m) User(id=2, age=28, name=nati)]
return nil
}

The full example exists in GitHub.

M2M Same Type#

er-following-followers

In this following-followers example, we have a M2M relation between users to their followers. Each user can follow many users, and can have many followers.

ent/schema/user.go

// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("following", User.Type).
From("followers"),
}
}

As you can see, in cases of relations of the same type, you can declare the edge and its reference in the same builder.

func (User) Edges() []ent.Edge {
return []ent.Edge{
+ edge.To("following", User.Type).
+ From("followers"),
- edge.To("following", User.Type),
- edge.From("followers", User.Type).
- Ref("following"),
}
}

The API for interacting with these edges is as follows:

func Do(ctx context.Context, client *ent.Client) error {
// Unlike `Save`, `SaveX` panics if an error occurs.
a8m := client.User.
Create().
SetAge(30).
SetName("a8m").
SaveX(ctx)
nati := client.User.
Create().
SetAge(28).
SetName("nati").
AddFollowers(a8m).
SaveX(ctx)
// Query following/followers:
flw := a8m.QueryFollowing().AllX(ctx)
fmt.Println(flw)
// Output: [User(id=2, age=28, name=nati)]
flr := a8m.QueryFollowers().AllX(ctx)
fmt.Println(flr)
// Output: []
flw = nati.QueryFollowing().AllX(ctx)
fmt.Println(flw)
// Output: []
flr = nati.QueryFollowers().AllX(ctx)
fmt.Println(flr)
// Output: [User(id=1, age=30, name=a8m)]
// Traverse the graph:
ages := nati.
QueryFollowers(). // [a8m]
QueryFollowing(). // [nati]
GroupBy(user.FieldAge). // [28]
IntsX(ctx)
fmt.Println(ages)
// Output: [28]
names := client.User.
Query().
Where(user.Not(user.HasFollowers())).
GroupBy(user.FieldName).
StringsX(ctx)
fmt.Println(names)
// Output: [a8m]
return nil
}

The full example exists in GitHub.

M2M Bidirectional#

er-user-friends

In this user-friends example, we have a symmetric M2M relation named friends. Each user can have many friends. If user A becomes a friend of B, B is also a friend of A.

Note that there are no owner/inverse terms in cases of bidirectional edges.

ent/schema/user.go

// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("friends", User.Type),
}
}

The API for interacting with these edges is as follows:

func Do(ctx context.Context, client *ent.Client) error {
// Unlike `Save`, `SaveX` panics if an error occurs.
a8m := client.User.
Create().
SetAge(30).
SetName("a8m").
SaveX(ctx)
nati := client.User.
Create().
SetAge(28).
SetName("nati").
AddFriends(a8m).
SaveX(ctx)
// Query friends. Unlike `All`, `AllX` panics if an error occurs.
friends := nati.
QueryFriends().
AllX(ctx)
fmt.Println(friends)
// Output: [User(id=1, age=30, name=a8m)]
friends = a8m.
QueryFriends().
AllX(ctx)
fmt.Println(friends)
// Output: [User(id=2, age=28, name=nati)]
// Query the graph:
friends = client.User.
Query().
Where(user.HasFriends()).
AllX(ctx)
fmt.Println(friends)
// Output: [User(id=1, age=30, name=a8m) User(id=2, age=28, name=nati)]
return nil
}

The full example exists in GitHub.

Edge Field#

The Field option for edges allows users to expose foreign-keys as regular fields on the schema. Note that only relations that hold foreign-keys (edge-ids) are allowed to use this option. Support for non-foreign-key fields in join tables is in progress (as of September 2021) and can be tracked with this GitHub Issue.

// Fields of the Post.
func (Post) Fields() []ent.Field {
return []ent.Field{
field.Int("author_id").
Optional(),
}
}
// Edges of the Post.
func (Post) Edges() []ent.Edge {
return []ent.Edge{
edge.To("author", User.Type).
// Bind the "author_id" field to this edge.
Field("author_id").
Unique(),
}
}

The API for interacting with edge-fields is as follows:

func Do(ctx context.Context, client *ent.Client) error {
p, err := c.Post.Query().
Where(post.AuthorID(id)).
OnlyX(ctx)
if err != nil {
log.Fatal(err)
}
fmt.Println(p.AuthorID) // Access the "author" foreign-key.
}

Multiple examples exists in GitHub.

Migration To Edge Fields#

As mentioned in the StorageKey section, Ent configures edge storage-keys (e.g. foreign-keys) by the edge.To. Therefore, if you want to add a field to an existing edge (already exists in the database as a column), you need to set it up with the StorageKey option as follows:

// Fields of the Post.
func (Post) Fields() []ent.Field {
return []ent.Field{
+ field.Int("author_id").
+ Optional(),
}
}
// Edges of the Post.
func (Post) Edges() []ent.Edge {
return []ent.Edge{
edge.From("author", User.Type).
+ Field("author_id").
+ StorageKey(edge.Column("post_author")).
Unique(),
}
}

Alternatively, this option can be configured on the edge-field instead:

// Fields of the Post.
func (Post) Fields() []ent.Field {
return []ent.Field{
+ field.Int("author_id").
+ StorageKey("post_author").
+ Optional(),
}
}

If you're not sure how the foreign-key was named before using the edge-field option, check out the generated schema description in your project: <project>/ent/migrate/schema.go.

Required#

Edges can be defined as required in the entity creation using the Required method on the builder.

// Edges of the Card.
func (Card) Edges() []ent.Edge {
return []ent.Edge{
edge.From("owner", User.Type).
Ref("card").
Unique().
Required(),
}
}

If the example above, a card entity cannot be created without its owner.

StorageKey#

By default, Ent configures edge storage-keys by the edge-owner (the schema that holds the edge.To), and not the by back-reference (edge.From). This is because back-references are optional and can be removed.

In order to use custom storage configuration for edges, use the StorageKey method as follows:

// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("pets", Pet.Type).
// Set the column name in the "pets" table for O2M relationship.
StorageKey(edge.Column("owner_id")),
edge.To("cars", Car.Type).
// Set the symbol of the foreign-key constraint for O2M relationship.
StorageKey(edge.Symbol("cars_owner_id")),
edge.To("friends", User.Type).
// Set the join-table, and the column names for a M2M relationship.
StorageKey(edge.Table("friends"), edge.Columns("user_id", "friend_id")),
edge.To("groups", Group.Type).
// Set the join-table, its column names and the symbols
// of the foreign-key constraints for M2M relationship.
StorageKey(
edge.Table("groups"),
edge.Columns("user_id", "group_id"),
edge.Symbols("groups_id1", "groups_id2")
),
}
}

Struct Tags#

Custom struct tags can be added to the generated entities using the StructTag method. Note that if this option was not provided, or provided and did not contain the json tag, the default json tag will be added with the field name.

// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("pets", Pet.Type).
// Override the default json tag "pets" with "owner" for O2M relationship.
StructTag(`json:"owner"`),
}
}

Indexes#

Indexes can be defined on multi fields and some types of edges as well. However, you should note, that this is currently an SQL-only feature.

Read more about this in the Indexes section.

Annotations#

Annotations is used to attach arbitrary metadata to the edge object in code generation. Template extensions can retrieve this metadata and use it inside their templates.

Note that the metadata object must be serializable to a JSON raw value (e.g. struct, map or slice).

// Pet schema.
type Pet struct {
ent.Schema
}
// Edges of the Pet.
func (Pet) Edges() []ent.Edge {
return []ent.Field{
edge.To("owner", User.Type).
Ref("pets").
Unique().
Annotations(entgql.Annotation{
OrderField: "OWNER",
}),
}
}

Read more about annotations and their usage in templates in the template doc.

Naming Convention#

By convention edge names should use snake_case. The corresponding struct fields generated by ent will follow the Go convention of using PascalCase. In cases where PascalCase is desired, you can do so with the StorageKey or StructTag methods.