Use

During all initializations of the connection Map type, if an id property exists then the constructor-hydrate behavior will check the connection instanceStore for a matching instance. If one is found it will be returned instead of a new object.

For example, this behavior is useful if Type converters of DefineMap are used in your app.

Let's say we have a page state type, two properties of which are of type Student:

var myPage = new (DefineMap.extend({
  student: { Type: Student },
  teamLead: { Type: Student },
  loadTeamLead: function() {...}
}));

The type Student is a DefineMap that has had a can-connect connection attached:

var dataUrl = require("can-connect/data/url/");
var constructor = require("can-connect/constructor/");
var store = require("can-connect/constructor/store/");
var canMap = require("can-connect/can/map/");
var constructorHydrate = require("can-connect/can/constructor-hydrate/");

var Student = DefineMap.extend({...});
Student.List = DefineList.extend({
    '#': { Type: Student }
});

Student.connection = connect(
  [dataUrl, constructor, store, canMap, constructorHydrate], {
       Map: Student,
       List: Student.List,
       url: "api/students"
  }
);

Now lets say your page loads myPage.student via the connection using Student.get(), and then it gets data for teamLead without the connection via myPage.loadTeamLead(). In this example it so happens that the team lead is the same person as the student:

Student.get({id: 1}).then(function(person) {
  // loaded via can-connect, person.id === 1
  myPage.student = person;

  // not loaded via can-connect, person.id === 1
  myPage.loadTeamLead(myPage.student.teamId).then(function(person){ myPage.teamLead = person; });
});

Without constructor-hydrate we would end up with two different instances of Student with the same id. Additionally, teamLead would not be an instance that is stored in the connection's instanceStore, and thus would not benefit from the real-time updates offered by the real-time behavior.

constructor-hydrate solves this problem by checking instanceStore before creating a new instance. So, in our example it will return the existing instance from myPage.loadTeamLead(). Now both myPage.student and myPage.teamLead are referencing the same instance:

var instanceStore = Student.connection.instanceStore;
myPage.student === myPage.teamLead;                           // => true
myPage.teamLead === instanceStore.get( myPage.teamLead.id );  // => true