var bookinghistory = database.ref('bookingHistory');
var hospital = database.ref('hospitals');
var client = database.ref('clients');
var patient = database.ref('patients');

bookinghistory.once('value', function (snapshot) {
        var promises = [];
        snapshot.forEach(function (childSnapshot) {

            var childKey = childSnapshot.key;
            var fk = snapshot.child(childKey).val();

            var myhospital = hospital.child(childSnapshot.val().hospital_fk).once('value');
            var myclient = client.child(childSnapshot.val().client_fk).once('value');
            var mypatient = patient.child(childSnapshot.val().patient_id).once('value');

            promises.push(new Promise((resolve, reject) => {
                 myhospital.then(docResult => {
                    resolve({
                        childKey: childKey,
                        bookhistory: fk,
                        hospital: docResult
                    });
                }).catch(reason => {
                    reject(reason);
                 });
             }));
            promises.push(new Promise((resolve, reject) => {
                 myclient.then(docResult => {
                    resolve({
                        client: docResult
                    });
                }).catch(reason => {
                    reject(reason);
                 });
             }));
            promises.push(new Promise((resolve, reject) => {
                 mypatient.then(docResult => {
                    resolve({
                        patient: docResult
                    });
                }).catch(reason => {
                    reject(reason);
                 });
             }));
       }));
});
Promise.all(promises).then(function(snapshots) {
        var dataSet = [];
         snapshots.forEach(function(hospital) {

            dataSet.push({
                childKey: hospital.childKey,
                booth: hospital.booth,
                hospital:hospital.hospital,
                client:hospital.client,
                patient:hospital.patient

            });

        });
        res.json(dataSet);
    });

[
    {
        "childKey": "-L2snm8lhye",
        "hospital": {
            "address": "Rizal Drive cor. ",
            "city": "",
            "createdAt": "2018-01-14 23:01:32",
            "deletedAt": "",
            "latitude": "14.554928",
            "longitude": "1115",
            "name": "St. Luke's Medical Center - Global City",
            "province": "",
            "updatedAt": ""
        }
    },
    {
        "client": {
            "createdAt": "2018-01-15",
            "firstName": "",
            "identifier": "+63933000000",
            "lastName": ""
        }
    },
    {
        "patient": {
            "birthdate": "1993-01-15",
            "client_fk": "qFUJT05B19NgQ2",
            "createdAt": "2018-01-15 15:02:13",
            "deletedAt": "",
            "firstName": "Eric Christian",
            "gender": "1",
            "lastName": "Odulio",
            "updatedAt": ""
        }
    }
]

[
    {
        "childKey": "-L2snm8lhye",
        "hospital": {
            "address": "Rizal Drive cor. ",
            "city": "",
            "createdAt": "2018-01-14 23:01:32",
            "deletedAt": "",
            "latitude": "14.554928",
            "longitude": "1115",
            "name": "St. Luke's Medical Center - Global City",
            "province": "",
            "updatedAt": ""
        }
        "client": {
            "createdAt": "2018-01-15",
            "firstName": "",
            "identifier": "+63933000000",
            "lastName": ""
        }
        "patient": {
            "birthdate": "1993-01-15",
            "client_fk": "qFUJT05B19NgQ2",
            "createdAt": "2018-01-15 15:02:13",
            "deletedAt": "",
            "firstName": "Eric",
            "gender": "1",
            "lastName": "Odulio",
            "updatedAt": ""
        }
   }
]