App crashes when I am trying to saved data in my model - ios

Question

I am developing an App using Realm. At some point in my app when I try to manipulate my model, my app crashed in an unexpected way. Here is what the stack trace said
Can only add, remove, or create objects in a Realm in a write transaction - call beginWriteTransaction on an RLMRealm instance first
What I am trying to do :
lets break down my problem in parts.following is my model of app
#objcMembers public class ClassGroup : Object , Codable {
dynamic var Id : Int? = ""
dynamic var ClassName : String? = ""
dynamic var TeacherId : Int = 0
dynamic var Teachers : [TeacherMdoel]? = []
}
#objcMembers public class TeacherModel : Object , Codable {
dynamic var Id : String? = ""
dynamic var Name : String? = ""
dynamic var ClassId : Int = 0
dynamic var Students : [StudentClass]? = []
}
#objcMembers public class StudentModel : Object , Codable {
dynamic var Id : String? = ""
dynamic var Name : String? = ""
dynamic var ClassId : Int = 0
dynamic var TeacherId : Int = 0
}
now I am trying to get the list of all classes like this from realm (after saving them to realm )
let mClassLists = mDbHelper.realmObj.objects(ClassGroup.self)
Now here I get exception/error. What I am doing is, I am trying to populate my UITableView with some data that consist of all of the above models. I am fetching data and saving them in my model and trying to supply that list to UITableView but my app crash with the error I mentioned above
let mClassLists = mDbHelper.realmObj.objects(ClassGroup.self)
let classLists = Array (mClassLists)
for classModel in classLists {
let resultPredicateTeachers = NSPredicate(format: "ClassId == %#", classModel.Id)
let mTeachersList = mDbHelper.realmObj.objects(TeacherModel.self).filter(resultPredicateTeachers)
if(mTeachersList.count > 0){
var listTeachers : [TeacherModel] = []
for teacherModel in mTeachersList {
let resultPredicateStudent = NSPredicate(format: "TeacherId == 29")
let mStudentList = mDbHelper.realmObj.objects(StudentModel.self).filter(resultPredicateStudent)
if(mStudentList.count > 0){
let studentsList = Array(mStudentList)
teacherModel.Students = studentsList[0]
}
listTeachers.append(savedDetailItem)
}
classModel.Teachers? = (listTeachers)
listClassModel.append(classModel)
}
}
**In the Above code you can see that I am gathering data on behalf of Ids and saving the resultant arrays in the model. So I am getting error in the following line
**
teacherModel.Students = studentsList[0]
now I really do not understand why it is happening? I am not saving data in realm, I am saving in my model, still I am getting error.

Answer 1

In Realm database, if you want to modify any model (save new data or update), the operation should be performed in a write transaction:
try! realm.write {
realm.add(<your_model_objects>)
}