Predicate types

A predicate is the smallest piece of information about an object. A predicate can hold a literal value or can describe a relation to another entity :

• when we store that an entity name is “Alice”. The predicate is name and predicate value is the string “Alice”.
• when we store that Alice knows Bob, we may use a predicate knows with the node representing Alice. The value of this predicate would be the [uid]/dgraph-glossary/#uid) of the node representing Bob. In that case, knows is a relationship.

Dgraph maintains a list of all predicates names and their type in the Dgraph schema.

A predicate type is either created

• by an alter operation (See Update Dgraph types ) or
• during a mutation : If a predicate type isn’t specified, then the type is inferred from the first mutation. If the mutation is using RDF format with an RDF type, Dgraph uses this information to infer the predicate type.

If no type can be inferred, the predicate type is set to default.

Scalar Types

For all triples with a predicate of scalar types the object is a literal.

UID Type

The uid type denotes a relationship; internally each node is identified by it’s UID which is a uint64.

Predicate name rules

Any alphanumeric combination of a predicate name is permitted. Dgraph also supports Internationalized Resource Identifiers (IRIs). You can read more in Predicates i18n.

Allowed special characters

Single special characters are not accepted, which includes the special characters from IRIs. They have to be prefixed/suffixed with alphanumeric characters.

][&*()_-+=!#$%

Note: You are not restricted to use @ suffix, but the suffix character gets ignored.

Forbidden special characters

The special characters below are not accepted.

^}|{`\~

Predicates i18n

If your predicate is a URI or has language-specific characters, then enclose it with angle brackets <> when executing the schema mutation.

Schema syntax:

<职业>: string @index(exact) .
<年龄>: int @index(int) .
<地点>: geo @index(geo) .
<公司>: string .

This syntax allows for internationalized predicate names, but full-text indexing still defaults to English. To use the right tokenizer for your language, you need to use the @lang directive and enter values using your language tag.

Schema:

<公司>: string @index(fulltext) @lang .

Mutation:

{
  set {
    _:a <公司> "Dgraph Labs Inc"@en .
    _:b <公司> "夏新科技有限责任公司"@zh .
    _:a <dgraph.type> "Company" .
  }
}

Query:

{
  q(func: alloftext(<公司>@zh, "夏新科技有限责任公司")) {
    uid
    <公司>@.
  }
}

Upsert directive

To use upsert operations on a predicate, specify the @upsert directive in the schema.

When committing transactions involving predicates with the @upsert directive, Dgraph checks index keys for conflicts, helping to enforce uniqueness constraints when running concurrent upserts.

This is how you specify the upsert directive for a predicate.

email: string @index(exact) @upsert .

Noconflict directive

The NoConflict directive prevents conflict detection at the predicate level. This is an experimental feature and not a recommended directive but exists to help avoid conflicts for predicates that don’t have high correctness requirements. This can cause data loss, especially when used for predicates with count index.

This is how you specify the @noconflict directive for a predicate.

email: string @index(exact) @noconflict .

RDF Types

Dgraph supports a number of RDF types.

As well as implying a schema type for a first mutation, an RDF type can override a schema type for storage.

If a predicate has a schema type and a mutation has an RDF type with a different underlying Dgraph type, the convertibility to schema type is checked, and an error is thrown if they are incompatible, but the value is stored in the RDF type’s corresponding Dgraph type. Query results are always returned in schema type.

For example, if no schema is set for the age predicate. Given the mutation

{
 set {
  _:a <age> "15"^^<xs:int> .
  _:b <age> "13" .
  _:c <age> "14"^^<xs:string> .
  _:d <age> "14.5"^^<xs:string> .
  _:e <age> "14.5" .
 }
}

Dgraph:

• sets the schema type to int, as implied by the first triple,
• converts "13" to int on storage,
• checks "14" can be converted to int, but stores as string,
• throws an error for the remaining two triples, because "14.5" can’t be converted to int.

Password type

A password for an entity is set with setting the schema for the attribute to be of type password. Passwords cannot be queried directly, only checked for a match using the checkpwd function. The passwords are encrypted using bcrypt.

For example: to set a password, first set schema, then the password:

pass: password .

{
  set {
    <0x123> <name> "Password Example" .
    <0x123> <pass> "ThePassword" .
  }
}

to check a password:

{
  check(func: uid(0x123)) {
    name
    checkpwd(pass, "ThePassword")
  }
}

output:

{
  "data": {
    "check": [
      {
        "name": "Password Example",
        "checkpwd(pass)": true
      }
    ]
  }
}

You can also use alias with password type.

{
  check(func: uid(0x123)) {
    name
    secret: checkpwd(pass, "ThePassword")
  }
}

output:

{
  "data": {
    "check": [
      {
        "name": "Password Example",
        "secret": true
      }
    ]
  }
}