DQL Type System

Dgraph supports a type system that can be used to categorize nodes and query them based on their type. The type system is also used during expand queries.

Type definition

Types are defined using a GraphQL-like syntax. For example:

type Student {
  name
  dob
  home_address
  year
  friends
}

Types are declared along with the schema using the Alter endpoint. In order to properly support the above type, a predicate for each of the attributes in the type is also needed, such as:

name: string @index(term) .
dob: datetime .
home_address: string .
year: int .
friends: [uid] .

Reverse predicates can also be included inside a type definition. For example, the type above could be expanded to include the parent of the student if there’s a predicate children with a reverse edge (the brackets around the predicate name are needed to properly understand the special character ~).

children: [uid] @reverse .

type Student {
  name
  dob
  home_address
  year
  friends
  <~children>
}

Edges can be used in multiple types: for example, name might be used for both a person and a pet. Sometimes, however, it’s required to use a different predicate for each type to represent a similar concept. For example, if student names and book names required different indexes, then the predicates must be different.

type Student {
  student_name
}

type Textbook {
  textbook_name
}

student_name: string @index(exact) .
textbook_name: string @lang @index(fulltext) .

Altering the schema for a type that already exists, overwrites the existing definition.

Setting the type of a node

Scalar nodes cannot have types since they only have one attribute and its type is the type of the node. UID nodes can have a type. The type is set by setting the value of the dgraph.type predicate for that node. A node can have multiple types. Here’s an example of how to set the types of a node:

{
  set {
    _:a <name> "Garfield" .
    _:a <dgraph.type> "Pet" .
    _:a <dgraph.type> "Animal" .
  }
}

dgraph.type is a reserved predicate and cannot be removed or modified.

Using types during queries

Types can be used as a top level function in the query language. For example:

{
  q(func: type(Animal)) {
    uid
    name
  }
}

This query will only return nodes whose type is set to Animal.

Types can also be used to filter results inside a query. For example:

{
  q(func: has(parent)) {
    uid
    parent @filter(type(Person)) {
      uid
      name
    }
  }
}

This query will return the nodes that have a parent predicate and only the parent’s of type Person.

You can also query for dgraph.type to get the Entity type. For example:

{
   people(func: eq(dgraph.type, "Person")) {
      name
      dgraph.type
   }
}

or

{
   people(func: type(Person)) {
      name
      dgraph.type
   }
}

Deleting a type

Type definitions can be deleted using the Alter endpoint. All that is needed is to send an operation object with the field DropOp (or drop_op depending on the client) to the enum value TYPE and the field ‘DropValue’ (or drop_value) to the type that is meant to be deleted.

Below is an example deleting the type Person using the Go client:

err := c.Alter(context.Background(), &api.Operation{
                DropOp: api.Operation_TYPE,
                DropValue: "Person"})

Expand queries and types

Queries using expand (e.g.: expand(_all_)) require that the nodes to be expanded have types.