Understanding Persistence Ignorance Principle

Before understanding what is Persistence Ignorance principle, we need to understand the difference between DTO and POCO.

 As a programmer we should know about DTOs represent as Data Transfer Objects A class object which contains state but not contains any behavior. It means a class contains only properties where it holds the data but it doesn’t contains any methods.

public class EmployeeDTO  
{  
    public String FirstName  
    public String LastName  
    public String Email  
    public String PhoneNo  
}

Similarly, POCO represents as Plain Old C# objects. A class object which contains both state and behavior is known as POCO. It means a class contains both properties declared to hold the state as well as methods to manage the behavior.

public class EmployeePOCO  
{  
    public String FirstName  
    public String LastName  
    public String Email  
    public String PhoneNo 
    public decimal GetLeavesCount() {} 
   }

Now, where does Persistence Ignorance principle suites here? Exactly, as we learn a POCO class can contain a behavior but as per this principle, this behavior shouldn’t be related to any persistence framework like Entity Framework, NHibernate etc..,

public class EmployeePOCO
{  
    public String FirstName  
    public String LastName  
    public String Email  
    public String PhoneNo 
    public decimal GetLeavesCount() {}  //This is not allowed as it should interact with persistence Framework  
}

It means, no method should be in a class which performs CRUD operations. This principle will be useful in DDD (Domain Driven Development) model development where we need to follow it to build Domain Model layer.