@property {Boolean} can-observable-object/define/identity identity
@parent can-observable-object/object.behaviors

Specifies that the property uniquely identifies instances of the type.

@type {Boolean}

  If `true`, specifies that the property uniquely identifies instances of the
  type.  `identity` configures the result of [can-reflect.getIdentity].

  The following specifies that the `id` property values uniquely identifies `Todo`
  instances:

  ```js
  import { ObservableObject, Reflect as canReflect } from "can/everything";

  class Todo extends ObservableObject {
    static props = {
      id: {
        type: Number,
        identity: true
      },
      name: String,
      complete: Boolean
    };
  }

  const todo = new Todo({id: 6, name: "mow lawn"});

  console.log( canReflect.getIdentity(todo) ); //-> 6
  ```
  @codepen

  `identity` can be `true` for multiple properties. If multiple identity properties
  are specified, a sorted JSON string is returned:

  ```js
  import { ObservableObject, Reflect as canReflect } from "can/everything";

  class Grade extends ObservableObject {
    static props = {
      classId: {
        type: Number,
        identity: true
      },
      studentId: {
        type: Number,
        identity: true
      },
      grade: String
    };
  }

  const grade = new Grade({classId: 5, studentId: 7, grade: "A+"});

  console.log( canReflect.getIdentity(grade) ); //-> "{'classId':5,'studentId':7}"
  ```
  @codepen


@body

## Use

`identity` is useful for models like [can-rest-model], [can-connect] and
[can-query-logic]. For example, it's used by [can-connect/can/map/map.prototype.isNew]
to know if the instance has been saved to the server or not.  It's used by [can-connect/real-time/real-time] to know which instance to update when [can-connect/real-time/real-time.updateInstance] is called.