I am studying JPA in Spring application and I have some doubts related to the #Entity annotation.
So I have a model class like this:
#Entity
#Table(name= “T_CUSTOMER”)
public class Customer {
#Id
#Column(name=“cust_id”)
private Long id;
#Column(name=“first_name”)
private String firstName;
#Transient
private User currentUser;
...........................
...........................
...........................
}
Ok, I know that the #Entity annotation is on the class level and it means that the fields of the object that are instances of this class are to be mapped to the field of the T_CUSTOMER database table.
But why in JPA it is mandatory to use #Entity annotation and I cannot only use the #Table annotation to map a model object to a specific database table? It have some other meaning\behavior that actually I am missing?
What am I missing? What is the exact meaning of the #Entity annotation?
#Entity annotation defines that a class can be mapped to a table. And that is it, it is just a marker, like for example Serializable interface.
And why #Entity annotation is mandatory? ... well, it is the way how JPA is designed. When you create a new entity you have to do at least two things
annotated it with #Entity
create an id field and annotate it with #Id
Anything else is optional, for example table name is derived from entity class name (and therefore #Table annotation can be optional), table's columns are derived from entities variables (and therefore #Column annotation can be optional), and so on ...
JPA is trying to provide a fast and easy start to developers who want to learn/use this API, and giving developers option to configure as few things as possible to make something functional is one of the ways how this API wants to achieve this "easy to use/learn" goal. Hence the #Entity annotation (together with #Id annotation) is the minimum you have to do in order to create an entity.
Entities in JPA are nothing but POJOs representing data that can be persisted to the database. An entity represents a table stored in a database. Every instance of an entity represents a row in the table.
More about the entities:
https://www.baeldung.com/jpa-entities
Entities represent persistent data stored in a relational database automatically using container-managed persistence.They are persistent because their data is stored persistently in some form of data storage system, such as a database: they do survive a server failure, failover, or a network failure. When an entity is reinstantiated, the state of the previous instance is automatically restored.
An entity models a business entity or multiple actions within a single business process. Entities are often used to facilitate business services that involve data and computations on that data. For example, you might implement an entity to retrieve and perform computation on items within a purchase order. Your entity can manage multiple, dependent, persistent objects in performing its tasks.
Entities can represent fine-grained persistent objects, because they are not remotely accessible components.
An entity can aggregate objects together and effectively persist data and related objects using the transactional, security, and concurrency services of a JPA persistence provider.
Related
DDD says that you domain models needs to be expressive and say what they need. With that in mind I created the following class:
#Entity
public class User{
#Id
#GeneratedValue
private Long id;
private String userName;
private String password;
public User(String userName, String password) {
/**set the values*/
}
/** Validations, getters and business methods*/
With this constructor I tell that the minimun needed to build a new User object.
The problem with that approach is that JPA needs a non-argument default constructor to instatiate the class by reflections. In this case do I have to forget about that DDD sugestion or is there a workaround?
In your specific situation you can make the non-argument constructor private. But soon another JPA requirement will force you to make another workaround;
voiceofunreason gave you the right answer. The right way is to have an adapter in the infrastructure/persistence layer that receives your domain object and then convert it to a JPA anemic model and vice versa.
Some other options that fits better without too much conversions:
1 - Spring Data JDBC is trying to absorb Aggregates concept
https://www.youtube.com/watch?v=GOSW911Ox6s&t=3s;
https://docs.spring.io/spring-data/jdbc/docs/1.1.7.RELEASE/reference/html/#reference
2 - Persist your aggregate as a json in a relational database. Vaughn Vernon proposed an implementation -> https://kalele.io/the-ideal-domain-driven-design-aggregate-store/
3 - Document oriented database such as https://www.mongodb.com/ among others...
In this case do I have to forget about that DDD sugestion or is there a workaround?
There isn't really a good answer here. There's a tension at work when we try to mix objects with mutable, encapsulated state, and persistence (ie: storing the authoritative copy of state in some anemic durable store).
The "right" answer is that we don't use an O/RM to load domain entities from the durable store; the O/RM loads a DTO, and we copy information from the DTO into the domain object. Similarly, to save, you extract information from the domain entity and copy it into the DTO, then save the DTO.
In other words, the O/RM loads an in memory representation of an anemic data structure.
In practice, it's a lot more common to couple the entity state to the O/RM, implementing within the domain entity whatever interfaces the O/RM needs to do its work.
The trick is to make sure that you can decouple your implementation from the O/RM when the O/RM assumptions start to get in the way. For instance, you should be able to easily change a specific aggregate from using a relational data store to using a document data store without requiring a rewrite of your entire system.
I would like to use one of the ideas that Jimmy Nilsson mentioned in his book Applying DDD with Patterns, and that is if i have an entity like a Product for example, i would like to take a snapshot of that entity for historic information, something like ProductSnapshot but i wonder how i might be able to implement this with an ORM (i am currently using Entity Framework). The main problem i am facing is that if i have another Entity like OrderLine that receives the Product via its constructor then entity framework would need you to make a public property of the type you wish to persist so this will force me to have something like this:
class OrderLine {
public Product Original Product {get; set;}
public ProductSnapshot Snapshot {get; set;}
}
and that seems awkward and not intuitive and i don't know how to deal with it properly when it comes to data binding (to which property i should bind), and finally i think that Product is an Entity while ProductSnapshot is a Value Object plus the snapshot is only taken when the OrderLine is accepted and after that the Product is not needed.
When doing DDD, forget that the database exists. This means the ORM doesn't exist either. Now, because you don't have to care about persistence and ORM limits, you can model the ProductSnapshot according to the domain needs.
Create a ProductSnapshot class with all the required members.This class would be a result probably of a SnapshotService.GetSnapshot(Product p) . Once you have the ProductSnapshot just send it to a repository SnapshotsRepository.Save(snapshot) . Being a snapshot, this means it will probably be more of a data structure, a 'dumb' object. It also should be invariable, 'frozen' .
The Repository will use EF to actually save the data. You decide what the EF entities and relations are. ProductSnapshot is a considered to be a business object by the persistence(it doesn't matter if in reality it's just a simple Dto) and the EF entities may look very different (for example, I store business objects in serialized form in a key-value table) according to your querying needs.
Once you define the EF entites you need to map the ProductSnapshot to them. It's very probable that ProductSnapshot itself can be used as an EF Entity so you don't need to do any mapping.
The point is, that taking a snapshot seems to be domain behavior. You deal with the EF only after you have the snapshot and you do exactly as you'd do with any other busines object.
Why does OrderLine have to have ProductSnapshot property? I suppose, you can either have a link to ProductSnapshot from Product class if you need to get that historical informatil, or, in case you just want to save Product state under some conditions, just implement a SaveSnapshot method in Product partial class, or have an extension method for it.
I have several concerns when trying to do DDD development with EF 4.2 (or EF 4.1) code first. I've done some extensive research but haven't come up with concrete answers for my specific concerns. Here are my concerns:
The domain cannot know about the persistence layer, or in other words the domain is completely separate from EF. However, to persist data to the database each entity must be attached to or added to the EF context. I know you are supposed to use factories to create instances of the aggregate roots so the factory could potentially register the created entity with the EF context. This appears to violate DDD rules since the factory is part of the domain and not part of the persistence layer. How should I go about creating and registering entities so that they correctly persist to the database when needed to?
Should an aggregate entity be the one to create it's child entities? What I mean is, if I have an Organization and that Organization has a collection of Employee entities, should Organization have a method such as CreateEmployee or AddEmployee? If not where does creating an Employee entity come in keeping in mind that the Organization aggregate root 'owns' every Employee entity.
When working with EF code first, the IDs (in the form of identity columns in the database) of each entity are automatically handled and should generally never be changed by user code. Since DDD states that the domain is separate from persistence ignorance it seems like exposing the IDs is an odd thing to do in the domain because this implies that the domain should handle assigning unique IDs to newly created entities. Should I be concerned about exposing the ID properties of entities?
I realize these are kind of open ended design questions, but I am trying to do my best to stick to DDD design patterns while using EF as my persistence layer.
Thanks in advance!
On 1: I'm not all that familiar with EF but using the code-first/convention based mapping approach, I'd assume it's not too hard to map POCOs with getters and setters (even keeping that "DbContext with DbSet properties" class in another project shouldn't be that hard). I would not consider the POCOs to be the Aggregate Root. Rather they represent "the state inside an aggregate you want to persist". An example below:
// This is what gets persisted
public class TrainStationState {
public Guid Id { get; set; }
public string FullName { get; set; }
public double Latitude { get; set; }
public double Longitude { get; set; }
// ... more state here
}
// This is what you work with
public class TrainStation : IExpose<TrainStationState> {
TrainStationState _state;
public TrainStation(TrainStationState state) {
_state = state;
//You can also copy into member variables
//the state that's required to make this
//object work (think memento pattern).
//Alternatively you could have a parameter-less
//constructor and an explicit method
//to restore/install state.
}
TrainStationState IExpose.GetState() {
return _state;
//Again, nothing stopping you from
//assembling this "state object"
//manually.
}
public void IncludeInRoute(TrainRoute route) {
route.AddStation(_state.Id, _state.Latitude, _state.Longitude);
}
}
Now, with regard to aggregate life-cycle, there are two main scenario's:
Creating a new aggregate: You could use a factory, factory method, builder, constructor, ... whatever fits your needs. When you need to persist the aggregate, query for its state and persist it (typically this code doesn't reside inside your domain and is pretty generic).
Retrieving an existing aggregate: You could use a repository, a dao, ... whatever fits your needs. It's important to understand that what you are retrieving from persistent storage is a state POCO, which you need to inject into a pristine aggregate (or use it to populate it's private members). This all happens behind the repository/DAO facade. Don't muddle your call-sites with this generic behavior.
On 2: Several things come to mind. Here's a list:
Aggregate Roots are consistency boundaries. What consistency requirements do you see between an Organization and an Employee?
Organization COULD act as a factory of Employee, without mutating the state of Organization.
"Ownership" is not what aggregates are about.
Aggregate Roots generally have methods that create entities within the aggregate. This makes sense because the roots are responsible for enforcing consistency within the aggregate.
On 3: Assign identifiers from the outside, get over it, move on. That does not imply exposing them, though (only in the state POCO).
The main problem with EF-DDD compatibility seems to be how to persist private properties. The solution proposed by Yves seems to be a workaround for the lack of EF power in some cases. For example, you can't really do DDD with Fluent API which requires the state properties to be public.
I've found only mapping with .edmx files allows you to leave Domain Entities pure. It doesn't enforce you to make things publc or add any EF-dependent attributes.
Entities should always be created by some aggregate root. See a great post of Udi Dahan: http://www.udidahan.com/2009/06/29/dont-create-aggregate-roots/
Always loading some aggregate and creating entities from there also solves a problem of attaching an entity to EF context. You don't need to attach anything manually in that case. It will get attached automatically because aggregate loaded from the repository is already attached and has a reference to a new entity. While repository interface belongs to the domain, repository implementation belongs to the infrastructure and is aware of EF, contexts, attaching etc.
I tend to treat autogenerated IDs as an implementation detail of the persistent store, that has to be considered by the domain entity but shouldn't be exposed. So I have a private ID property that is mapped to autogenerated column and some another, public ID which is meaningful for the Domain, like Identity Card ID or Passport Number for a Person class. If there is no such meaningful data then I use Guid type which has a great feature of creating (almost) unique identifiers without a need for database calls.
So in this pattern I use those Guid/MeaningfulID to load aggregates from a repository while autogenerated IDs are used internally by database to make a bit faster joins (Guid is not good for that).
I am confused to understand what is the meaning of this words:
Entity, Model, DataModel, ViewModel
Can any body help me to understanding them please? Thank you all.
The definition of these terms is quite ambiguous. You will find different definitions at different places.
Entity: An entity represents a single instance of your domain object saved into the database as a record. It has some attributes that we represent as columns in our tables.
Model: A model typically represents a real world object that is related to the problem or domain space. In programming, we create classes to represent objects. These classes, known as models, have some properties and methods (defining objects behavior).
ViewModel: The term ViewModel originates from the MVVM (Model View ViewModel) design pattern. There are instances in which the data to be rendered by the view comes from two different objects. In such scenarios, we create a model class which consists of all properties required by the view. It’s not a domain model but a ViewModel because, a specific view uses it. Also, it doesn’t represent a real world object.
DataModel: In order to solve a problem, objects interact with each other. Some objects share a relationship among them and consequently, form a data model that represents the objects and the relationship between them.
In an application managing customer orders, for instance, if we have a customer and order object then these objects share a many to many relationship between them. The data model is eventually dependent on the way our objects interact with each other. In a database, we see the data model as a network of tables referring to some other tables.
To know more about object relationships visit my blog post: Basics of Object Relationships
For more details visit my blog post: Entity vs Model vs ViewModel vs DataModel
I hope I've not missed your point here king.net...
Anyway, presuming you're talking about entity modelling or entity-relationship modelling (ERDs):
an entity represents any real world entity - e.g. student, course,
an entity will have attributes - e.g. student has first name, surname, date-of-birth
an entity will have relationships - e.g. student "is enrolled on" course (where student and course are entities with attributes and "is enrolled on" is the relationship.
the relationship may be "one-to-one", "one-to-many" or "many-to-many" - e.g. one student "is enrolled on" many courses and similarly one course "has" many students.
relationships also have cardinality
Adding relationships between entities creates a "data model". You've modeled some real world system and the internal entities/ objects in that system. Next step is to normalise it to ensure it meets "normal form".
In ERD terms, you may have "logical" and "physical" models. The logical describes the data-model in simple high-level terms that witholds the technical detail required to implement it. It represents the system solution overview. The physical model includes technical details required to actually implement the system (such as "many-to-many join tables" needed to implement "many-to-many" relationships).
Here are some tutorials on-line (though I'm sure there must be thousands):
http://www.maakal.com/maakalDB/Database101ERDpart1.htm
http://www.itteam-direct.com/gwentrel.htm
http://www.bkent.net/Doc/simple5.htm
I'm not quite sure what you mean by "model" and "view model" in a related context. Not sure if you may be confusing this with Model-View-Controller paradigm (MVC). Here, a model is some data component and the view represents an observer of that data (such as a table or graph UI component). There's lots on-line explaining "model view controller" or "MVC".
Hope this helps, Wayne
Entity:
An entity is the representation of a real-world element within Object Relational Mapping (ORM) as the Entity Framework. This representation will be mapped to a table in a database and its attributes will be transformed into columns. An entity is written using a POCO class that is a simple class, as you can see in the following example in C#:
using System;
using System.Collections.Generic;
using System.Text;
namespace MyAplication.Entity
{
public class Person
{
public long PersonId { get; set; }
public string Name { get; set; }
public short Age { get; set; }
}
}
Working with UI creation is a complex task. To keep things organized, programmers separate their applications into layers.
Each layer is responsible for a task and this prevents the code from being messed up. It is in this scenario that the architectural patterns like the MVC and the MVVM appear.
Model:
Within the MVC we have a layer responsible for representing the data previously stored, a given could be an instance of a person modeled in the previous example. This layer is the Model. This template will be used to construct the view.
ViewModel:
A ViewModel in the MVVM architecture is much like a Model in the MVC architecture. However a ViewModel is a simplified representation of the data with only the information that is required for the construction of a view.
using System;
using System.Collections.Generic;
using System.Text;
using MyAplication.Web.ViewModel.BaseViewModel;
namespace MyAplication.Web.ViewModel.Person
{
public class PersonNameViewModel : BaseViewModel<string>
{
//I just neet the name
public string Name { get; set; }
}
}
DataModel:
It is simply an abstract model (this model is different from the MVC layer model) which establishes the relationships that exist between the elements that represent real-world entities. It is a very comprehensive subject.
First of all,to know about Entity you must know about Class.
All of them represent same fields but the terminology changes based on declaration.
Let us consider a table from any database[SQL,ORACLE,Informix,Cassandra..] as example.
CLASS:
Generally a table is a considered as a class until it is added to edmx or dbmx.
//Student class
public class Student()
{
//Properties
public int StudentNumber;
public string StudentName;
}
ENTITY:
After drag drop/adding the table into dbmx/edmx it is referred to as
Entity.
Each Entity is generated from its corresponding class and we can add
attributes to entity which are used for performing operations using
linq or entity.
DATAMODEL:
Contains all the fields in table.
DATAMODEL is a direct class reference to your cshtml or controller
where you can access the attributes to perform CRUD operations.
VIEWMODEL:
Some situations occur where we need to perform CRUD operations more
than one model(table).
So we combine all our required models in a class and define them in
its constructor.
Example:
Lets assume
//Student class
public class Student()
{
//Properties
public int StudentNumber;
public string StudentName;
}
//Marks Class
Public class Marks()
{
public int Maths;
public int Physics;
public int Chemistry;
//Now sometimes situations occur where we have to use one datamodel inside //other datamodel.
public Student StudentModel;
}
Simple talk:
DTO stands for Data Transfer Object. DTOs are mainly used for transferring data between services (web services, APIs, etc.) which can encompass variety of properties of different entities (with or without their ID). Take this row as an example of a DTO: Consider that a shopping website is going to send its shipping requests to a shipping company by a web-service. Its DTO would be something like this: CustomerFullName, ShippingFee, ShippingAddress. In this example CustomerFullName is combination of properties FirstName + LastName for the Customer entity, and ShippingFee is the result of several processes of destination, tax, etc over some other entities.
On the contrary, Entities are bunch of properties gathered to represent a single entity with a specific ID (e.g., Teacher, Student, Employee, etc.). In other words, DTOs are a bunch of meaningless properties gathered to be sent to the client and a DTO doesn't necessarily have relationship to the other DTOs, while an Entity includes properties of a specific object with meaningful relation to the other entities. In a relational database paradigm, we can look at DTOs as views' row while Entities are tables' row with the primary key.
However, Model is a combination of these two. A model can contain several related entities plus extra data to handle real-world application/UI problems. Consider a Model named CustomerOrdersModel that contains Customer Entity, List<Order> Entities, and an extra Boolean flag PayWithCredit specifying whether user is going to pay with debit-card or credit-card.
I need to store a large amount of nodes with many-to-many relationship to eachother (through parents/childrens). How can I do that in JPA?
I know I can use #ManyToMany on my parents/childrens list but then JPA will load the all parents and their parents and so on right? Is there any way I can do it and only load the parents or children one step away?
The non-working code I have now (the Node.java):
public String name;
#ManyToMany
#JoinTable(name = "NodeParent")
public List<Node> parents;
#ManyToMany
#JoinTable(name = "NodeChilds")
public List<Node> childrens;
For this JPA has the concept op lazy loading.
While iterating over the collection when still within the persistence context, every time you reference a not previously loaded entity, it will be loaded on demand.
Additionally, if you know up-front which entities you need (how deep you want the tree to be fetched) you can write a projection query (JPQL) using the fetch join operator.
Finally, a hibernate specific solution that might be included in JPA 2.1 are so-called fetch profiles. See Fetch profiles and Fetch Profiles in Hibernate 3.5.