association, aggregation and composition
I want to get the illustration for above three with simple classes. I read a lot from internet. The conclusion is as-
In aggregation people say-
"Class A contains collection of another Class (say B) and if A is destroyed it will not affect its child that is collection will not be destroyed."
How is it possible if one object is destroyed but its property can still exist or what they mean by this.(Am I misinterpreting something)
Class A
{
List<B> lst;
}
Class B
{
}
Consider the following classes,
class Student
{
public string Id { get; set; }
public string Name { get; set; }
}
class Department
{
public IList<Student> Students { get; set; }
public void AddStudent(Student student)
{
//...
}
public void RemoveStudent(Student student)
{
//...
}
}
If you want to add a student to the department, then you call AddStudent() and pass the reference of the Student class instance (note that a reference is passed). So when the department instance is destroyed (set to null for example), then the Students property of that Department instance is no longer available, but the Student instances that have been used to populate this list remain are not destroyed. Hence, the property, in this case a Student instance can still exist.
More information
implementation of composition and aggregation in C#?
C# Tutorial - Aggregation and Composition
Difference between Composition and Aggregation
Related
I am building a Web API using Dapper for .NET Core and trying to adhere to Clean Architecture principles. The API is consumed by an external Angular front-end.
I have repositories that use Dapper to retrieve data from the database, and this data then passes through a service to be mapped into a DTO for display to the user.
It is my understanding that an entity should be an exact representation of the database object, with no extra properties, and that I should use DTOs if I require some additional properties to show the user (or if I wish to obscure certain properties from the user too).
Suppose I have a DTO:
public class StudentDTO
{
public Guid Id { get; set; }
public string Name { get; set; }
public List<Assignment> Assignments { get; set;}
}
and its corresponding Entity:
public class Student
{
public Guid Id { get; set; }
public string Name { get; set; }
}
With this model, should I want to get a student with all of their assignments, I'd need to have two repository calls, and do something like this in the service:
public StudentDTO GetById(Guid id)
{
var student = this.studentRepository.GetById(id);
var assignments = this.assignmentRepository.GetByStudentId(id);
return SomeMapperClass.Map(student, assignments);
}
But this seems inefficient and unnecessary. My question is, should I not just retrieve the Assignments when I get the student entity in the repository, using a JOIN? Or would this violate what an entity is supposed to be?
I apologise, I do realise this is a rather simple question, but I'd really like to know which method is the best approach, or if they both have their use cases
I think it would be more efficient, since map uses reflections, that is slower tens times
public StudentDTO GetById(Guid id)
{
var student = this.studentRepository.GetById(id);
student.Assignments = this.assignmentRepository.GetByStudentId(id);
return student;
}
but the common way is
return _context.Students.Include(i=>i.Assignments).FirstOrDefault(i=> i.Id==id);
This is why the generic repository is a bad idea in the most casses, since it is hard to guess what set of data you will need.
What is the best way to do the following:
Suppose I have a class called Person and many derived classes for specialized persons.
Suppose at the beginning of my app, I know I have to deal with a person but I won't know what kind of person it is until much later (something beyond my control so I cannot determine the Person type at the beginning).
So at the beginning I will create a Person and fill in attributes for it. Later, when I know what kind of Person it is, I would instantiate a specialized person and copy over the any saved attributes for her.
Is there a more elegant way to do this without creating two objects?
If you don't know the type of person up front, you won't be able to avoid instantiating two objects. There has to be something to contain the base Person attributes before you know the specialized person, but you can't take advantage of polymorphism without instantiating the specialized object later.
One option is to use a composition pattern, in which each specialized person contains a Person instance rather than inheriting from it. You still have to instantiate two objects, but you don't have to rewrite the code to copy over the saved attributes every time. Here's an example (C# syntax):
public interface IPerson
{
string Name { get; }
int Age { get; }
}
public class Person : IPerson
{
public string Name { get; private set; }
public int Age { get; private set; }
public Person(string name, int age)
{
Name = name;
Age = age;
}
}
public abstract class SpecialPersonBase : IPerson
{
private IPerson myPerson;
protected SpecialPersonBase(IPerson person)
{
myPerson = person;
}
public string Name { get { return myPerson.Name; } }
public int Age { get { return myPerson.Age; } }
public abstract string Greet();
}
public class Doctor : SpecialPersonBase
{
public Doctor(IPerson person) : base(person) { }
public override string Greet()
{
return "How are you feeling?";
}
}
public class Accountant : SpecialPersonBase
{
public Accountant(IPerson person) : base(person) { }
public override string Greet()
{
return "How are your finances?";
}
}
You could use the classes like this:
IPerson bob = new Person("Bob", "25");
// Do things with the generic object
// until you can determine the specific type
SpecialPerson specialBob;
if (bobIsDoctor)
{
specialBob = new Doctor(bob);
}
else if (bobisAccountant)
{
specialBob = new Accountant(bob);
}
specialBob.Greet();
I'm designing a layered architecture (Service/Business Logic Layer, Data Access Layer) and am struggling with the intersection of a few problems.
Entity Framework 4.1 does not support interfaces directly
My Interfaces contain collections of other interfaces with read/write properties
This means using an implementing class won't work either, since it would still refers to another interface type
Example (please excuse the poorly written code, this is ad-hoc from my brain):
Data Access Layer
public interface IEmployer
{
string Name { get; set; }
ICollection<IEmployee> Employees { get; set; }
}
public interface IEmployee
{
string Name { get; set; }
}
public class Employer : IEmployer
{
public string Name { get; set; }
public ICollection<IEmployee> Employees { get; set; }
}
public class Employee : IEmployee
{
public string Name { get; set; }
}
public class DataManager
{
public IEmployer GetEmployer(string name) { ... }
public IEmployee CreateEmployeeObject(string name) { ... }
public void Save(IEmployer employer) { ... }
public void Save(IEmployee employee) { ... }
}
Service Layer
[DataContract]
public class Employee
{
[DataMember]
public string Name { get; set; }
}
public class HireService
{
public void HireNewEmployee(Employee newEmployee, string employerName)
{
DataManager dm = new DataManager();
IEmployer employer = dm.GetEmployer(employerName);
IEmployee employee = dm.CreateEmployeeObject(newEmployee.Name);
dm.Save(employee);
employer.Employees.Add(employee);
dm.Save(employer);
}
}
Without EF, the above works fine. The IEmployee type is used in the service layer, and does not conflict with the Employee data contract type. However, EF cannot use an interface, so I would be required to use class instead of an interface.
I see a few options:
Change IEmployer/IEmployee to classes, leaving the same names
Change IEmployer/IEmployee to classes, rename to EmployerDAL/EmployeeDAL
Change IEmployer/IEmployee to classes, rename to Employer/Employee, sprinkle using EmployerDL = DataLayer.Employer at the beginning of any service classes using it
What naming convention should I follow for class names which are defined in both the business and data layer?
Similar question to this: What's the naming convention for classes in the DataAccess Project? except that EF causes a problem with interfaces.
Actually the class defined in your DAL should be the one used in your business layer - those are your real domain objects. Classes exposed from your business layer are just data transfer objects so if you want to build any convention you should imho rename your data contracts.
Anyway the naming convention is something really subjective. Choose the way which best fits your needs and be consistent in that naming.
I have the following classes:
class Employee
{
public string Name { get; set; }
}
class HistoricalEmployee : Employee
{
public DateTime TerminationDate { get; set; }
}
class Company
{
public IList<Person> CurrentEmployees { get; set; }
}
Employee and HistoricalEmployee are mapped using table-per-class-heirarchy strategy.
When I retrieve the CurrentEmployees collection, I want it only to contain elements that are Employee, and NOT HistoricalEmployees.
when an employee 'dies', they're not really deleted, but they become HistoricalEmployee (with a few more attributes, such as termination date etc.).
Obviously, over time, the number of HistoricalEmployees will exceed the number of Employees by magnitudes, so I can't fetch all HistoricalEmployees when I only need current Employees.
How can I (fluently) configure the collection to only retrieve elements of the super class?
I think it's something to do with the Polymorphism property, but I couldn't really figure out how to do that.
thanks,
Jhonny
Ok, I did this like so:
mapping.HasMany(x => x.CurrentEmployees)
//.Where(pqa => pqa.TimeOut != null)
.Where("TerminationDate is null")
apparently, the .Where() function creates a filter on the property, which is exactly what I needed.
notice that I used the string version, and commented-out the Func<> version.
This is because that currently (FNH 1.1), as far as I could determine, the Func<> version doesn't work.
hopes this helps somebody,
J
I'm using two class NiceCustomer & RoughCustomer which implment the interface ICustomer.
The ICustomer has four properties. They are:
Property Id() As Integer
Property Name() As String
Property IsNiceCustomer() As Boolean
ReadOnly Property AddressFullText() As String
I don't know how to map the interface ICustomer, to the database.
I get an error like this in the inner exception.
An association refers to an unmapped class: ICustomer
I'm using Fluent and NHibernate.
You can map directly to interfaces in NHibernate, by plugging in an EmptyInterceptor during the configuration stage. The job of this interceptor would be to provide implementations to the interfaces you are defining in your mapping files.
public class ProxyInterceptor : EmptyInterceptor
{
public ProxyInterceptor(ITypeHandler typeHandler) {
// TypeHandler is a custom class that defines all Interface/Poco relationships
// Should be written to match your system
}
// Swaps Interfaces for Implementations
public override object Instantiate(string clazz, EntityMode entityMode, object id)
{
var handler = TypeHandler.GetByInterface(clazz);
if (handler == null || !handler.Interface.IsInterface) return base.Instantiate(clazz, entityMode, id);
var poco = handler.Poco;
if (poco == null) return base.Instantiate(clazz, entityMode, id);
// Return Poco for Interface
var instance = FormatterServices.GetUninitializedObject(poco);
SessionFactory.GetClassMetadata(clazz).SetIdentifier(instance, id, entityMode);
return instance;
}
}
After this, all relationships and mappings can be defined as interfaces.
public Parent : IParent {
public int ID { get; set; }
public string Name { get; set; }
public IChild Child { get; set; }
}
public Child : IChild {
public int ID { get; set; }
public string Name { get; set; }
}
public class ParentMap : ClassMap<IParent>
{
public ParentMap()
{
Id(x => x.ID).GeneratedBy.Identity().UnsavedValue(0);
Map(x => x.Name)
}
}
...
This type of technique is great if you want to achieve true decoupling of your ORM, placing all configuration/mappings in a seperate project and only referencing interfaces. Your domain layer is then not being polluted with ORM, and you can then replace it at a later stage if you need to.
how are you querying? If you're using HQL you need to import the interface's namespace with an HBM file with this line:
<import class="name.space.ICustomer, Customers" />
If you're using Criteria you should just be able to query for ICustomer and it'll return both customer types.
If you're mapping a class that has a customer on it either through a HasMany, HasManyToMany or References then you need to use the generic form:
References<NiceCustomer>(f=>f.Customer)
If you want it to cope with either, you'll need to make them subclasses
Subclassmap<NiceCustomer>
In which case I think you'll need the base class Customer and use that for the generic type parameter in the outer class:
References<Customer>(f=>f.Customer)
Regardless, you shouldn't change your domain model to cope with this, it should still have an ICustomer on the outer class.
I'm not sure if the 1.0RTM has the Generic form working for References but a quick scan of the changes should show the change, which I think is a two line addition.
It is not possible to map an interface in nhibernate. If your goal is to be able to query using a common type to retrieve both types of customers you can use a polymorphic query. Simply have both your classes implement the interface and map the classes normally. See this reference:
https://www.hibernate.org/hib_docs/nhibernate/html/queryhql.html (section 11.6)