DocumentId

@Retention(value = RetentionPolicy.RUNTIME)
@Target(value = [ElementType.FIELD, ElementType.METHOD])
public annotation DocumentId

Annotation used to mark a POJO property to be automatically populated with the document's ID when the POJO is created from a Cloud Firestore document (for example, via toObject).

Any of the following will throw a runtime exception:

  • This annotation is applied to a property of a type other than String or DocumentReference.
  • This annotation is applied to a property that is not writable (for example, a Java Bean getter without a backing field).
  • This annotation is applied to a property with a name that conflicts with a read document field. For example, if a POJO has a field `firstName` annotated by @DocumentId, and there is a property from the document named `firstName` as well, an exception is thrown when you try to read the document into the POJO via toObject or get.

When using a POJO to write to a document (via set or @set), the property annotated by @DocumentId is ignored, which allows writing the POJO back to any document, even if it's not the origin of the POJO.

Kotlin Note

When applying this annotation to a property of a Kotlin class, the @set use-site target should always be used. There is no need to use the @get use-site target as this annotation is only considered when reading instances from Firestore, and is ignored when writing instances into Firestore.

Here is an example of a class that can both be written into and read from Firestore whose foo property will be populated with the Document ID when being read and will be ignored when being written into Firestore: 

data class Pojo(@set:DocumentId var foo: String? = null) {
  constructor() : this(null) // Used by Firestore to create new instances
}