I'm new in the work with the Object Relational Mapping Tool's and I want to know what has caused the programming tools are created. What a privilege to work with the database and mapping tools will be for us?
ORMs are used to close the gap between object-oriented models (applications) and the relational model (database). There are several difficulties for that mapping, like inheritance and object identification to name only a few (for example, check out http://www.cit.dk/cot/reports/reports/Case4/05-v1.1/cot-4-05-1.1.pdf to go on here).
There are many ORM implementations/frameworks that use different concepts to achieve the mapping and they offer different 'features'.
As you might know there are object-oriented database-systems too, but the relational model is still favored because of practical issues. For your application you could map the datamodel
on your own or (if available and it fits your needs) use an ORM.
Related
In my spare time, I'm trying restart my effort to learn F#. I'm doing so by trying to create a simple application that will allow me to analyze my financial transactions.
My first attempt at creating this application failed due to the persistence step. I used SQL and the EntityFramework package, but the latter generated database entities, which I did not want to use throughout my application since they're all mutable (I think..). Instead I had to map these database entities to domain entities. Much manual glue code later it worked....until I found a bug and was forced to replace much of that glue code. That was the tipping point that made me quit.
On SO I found a question describing my situation, e.g. Saving F# types to a database. Mark Seeman suggested that the pain of mapping can be overcome if I'd not use objects for persistence. At work I have recently been introduced to MongoDb, which at least saves me the pain of mapping from database entities to domain entities. These entities all need some ID, and I chose to use an ObjectId from Mongo. Ooops, there comes the deja vu, in order not to have my domain entities being dependent on Mongo, I will once more have to create database and domain entities....as well as the mapping. Bah & Ugh.
In C# I'm used to do such mapping with tools like Automapper, but they don't really work for special F# types. So now I'm wondering what Mark Seeman ment by "using objects for persistance". How is this solved in F#? So far I haven't been able to fine more info on this topic besides the aforementioned question on SO.
I have started using Entity Framework Code First for the first time and am impressed by the way in which our greenfield application is being built around the domain rather than around the relational database tables (which is how I have worked for years).
So, we are building entities in C# that are being reflected in the database every time we do a new migration.
My question is this: should these same entities (i.e. designed with Entity Framework in mind) play the same role as entities in Domain Driven Design (i.e. representing the core of the domain)?
Object-Relational Mapping and Domain-Driven Design are two orthogonal concerns.
ORM
An ORM is just here to bridge the gap between the relational data model residing in your database and an object model, any object model.
An Entity as defined by EF concretely means any object that you wish to map some subpart of your relational model to (and from). It turns out that the EF creators wanted to give a business connotation to those by naming them Entities, but in the end nothing forces you that way. You could map to View Models for all it cares.
DDD
From a DDD perspective, there's no such thing as "an Entity designed with EF in mind". A DDD Entity should be persistence ignorant and bear no trace of any ORM. The domain layer has no interest in how, where, whether or when its objects are stored.
Where the two meet
The only point where the two orthogonal concepts intersect is when the object model targeted by your ORM mapping is precisely your domain model. This is possible with what EF calls "Code first" (but should really be named regular ORM), by pointing to your DDD Entities in separate EF mapping files living in a non-domain layer, and refraining from using EF artifacts such as data annotations directly in your Entity classes. This is not possible when using Database First, because the DDD "purity" part of the deal wouldn't be met.
In short, the terms collide, but they should really be conceptually considered as two different things. One is the domain object itself and the other is a pointer that can indicate the same bunch of code, but it could point to pretty much anything else.
They shouldn't be the same as they're designed for different purposes. An ORM entity is a facade for 1 or more tables, its purpose is to simulate OOP on top of relational tables. A Domain Entity is about defining a Domain concept. If your Domain Entity turns out to be just a data structure, then you can reuse it as an EF entity, but that's just one case.
A DDD app never knows about EF or ORM. It only knows about a Repository. Hence, your Domain Objects (DO) don't know either about EF. You can choose to consider them EF entities, as an implementation detail, BUT... you should do that ONLY after your DOs are defined and their use cases implemented. You should defer as much as possible the implementation of persistence (use in-memory repos (lists) for devel).
When you reach that point you'll know if you can reuse your DO for ORM purposes or if you'll need other ways (such as a memento).
Note that a design of a DO while driven by the Domain, it should take into consideration the persistence issue, but it shouldn't be influenced by it i.e don't design your DO according to the db schema. The persistence strategy can be different for each DO and it might involve or not an ORM.
If you're using Event Sourcing for a DO, ORM doesn't exist. Same for serialized objects. It matters a lot how an object will be used by the app (updating and querying), that's why I've said you should defer the persistence implementation. For a lot of DOs you won't need a rdbms (even if you're using it) so an ORM entity will look more like a KeyValuePair (Id => serialized data).
In conclusion, they are different things for different purposes, that might look identical for some cases (CRUD scenarios).
I would say, they can be the same.
Sometimes there is no need to support two models. When you follow code first approach, your entities model your domain, your infrastructure (ORM) separates domain and persistence layers.
It might be reasonable to maintain two models if you have legacy database and have to maintain it.
There are two other SO questions that can be helpful:
Repository pattern and mapping between domain models and Entity Framework
Advice on mapping of entities to domain objects
Well.That's The Approach i use.And I've seen a lot of others doing the same.Now am using The Onion Architecture/Pattern to Create my application and making Everything rely on the domain entities made my life easier.because whenever i want to change for example the Layer that deal with my database ,i can do that without changing the UI layer(ASP.NET MVC app,WPF app...etc)...I suggest doing the same.
let's wait for other posts
I agree with what MikeSW said (3rd Answer).When you design your domain entities,you should do that without caring about who will consume those entities (ORMs or any other technology serving whatever purpose).design them with one idea in mind : they will be reusable and they will not need to be changed in the future (hopefully)
I understand that at least EF 6 supports multiple DbContexts. This can be used to model BoundedContext. I did some google searches but could not find a definitive answer to this question. Is it advisable to use different db schemas for different DbContexts/BoundedContext? I know that ORMs abstract away the persistence mechanisms but I personally can see parallels between shemas and ddd/ef contexts.
It is a possibility. As with most architectural questions, the answer is: it depends.
In this case, it depends on how your overall architecture is and how your bounded contexts are structured. If they have similar aggregates that are persisted to the same tables (that is, they're different because of the context), it might be a good idea to have different DbContexts because then you can evolve them separately.
Note though that you may be introducing hidden constraints and dependencies between your bounded contexts.
If your bounded contexts have very different aggregates, then there's no need to use different DbContexts and you can just share the same one.
Another option you might consider, is using a different DbContext for reading and writing. It also allows you to evolve your model separately. (that's more of a CQRS approach though)
I stumbled upon the following two articles First and Second in which the author states in summary that ORM Entities and Domain Entities shouldn't be mixed up.
I face exactly this problem at the moment as I code with EF 6.0 using the Code First approach. I use the POCO classes as entities in the EF as well as my domain/business objects. But I find myself frequently in the situation where I define a property as public or a navigation property as virtual only because the EF Framework forces me to do so.
I don't know what to take as the bottom line of the two articles? Should I really create for example a CustomerEF class for the entity framework and a CustomerD for my domain. Then create a repository which consumes CustomerD maps it to CustomerEF do some queries and than maps back the received CustomerEF to CustomerD. I thought EF is all about mapping my domain entities to the data.
So please give me some advice. Do I overlook an important thing the EF is able to provide me with? Or is this a problem which can not completely solved by the EF? In the latter case what is a good way to manage this problem?
I agree with the general idea of these posts. An ORM class model is part of a data access layer first and foremost (even if it consists of so-called POCOs). If any conflict of interests arises between persistence and business logic (or any other concern), decisions should always be made in favor of persistence.
However, as software developers we always have to balance between purism and pragmatism. Whether or not to use the persistence model as a domain model depends on a number of factors:
The size/coherence of the development team. When the whole team knows that properties can be public just because of ORM requirements, but should not be set all over the place, it may not be a big deal. If everybody knows (and obeys) that an ID property is not to be used in business logic, having IDs may not be a big deal. A scattered, unexperienced or undisciplined team may need more stringent segregation of code.
The overlap between business logic concerns and persistence concerns. Object oriented design thrives when a class model sticks to SOLID principles. But these principles are not necessarily at odds with persistence concerns. I mean that although the concerns are different, in the end their resultant requirements may be quite similar. For instance, both concerns may require valid object state and correct associations.
There can be use cases, however, in which objects temporarily need to be in a state that absolutely shouldn't be stored. This may be a reason to work with dedicated domain classes. Another reason may be that the entity model just can't fulfill the best segmentation of responsibilities. For instance, a business process "blacklisting customer" may require data that is scattered over so many entity objects that new domain classes must be designed that can encapsulate the data and the methods working on them. In other words: doing this by entities would violate the Tell Don't Ask principle.
The need for layering. For instance, if the data access layer targets different database vendors it may have to consist of interchangeable parts that are vendor-specific (e.g. to account for subtle differences in data types between Oracle and Sql Server or to exploit vendor-specific features). Using the persistence model as domain model would probably bleed vendor-specific implementations into the business logic. That would be really bad. There the data access layer should be precisely that, a layer.
(Very trivial) The amount of data. Creating objects takes time and resources. When "many" objects are involved in a business case it may just be too expensive to build both entity objects and domain objects.
And more, undoubtedly.
So I would always try to be a pragmatist. If entity classes do a decent job, go for it. If the mismatch is too large, create a business domain for appropriate parts of the business logic. I would not slavishly follow a (any) design pattern just because it is a good pattern. Contrary to what is said in the post, it requires a lot of maintenance to map an entity model onto a business model. When you find yourself creating myriads of business classes that are almost identical to entity classes it's time to rethink what you're doing.
Let's say I have an app that should ideally be able to use a relational database, object database, XML files, or whatever to persist its data. In the spirit of coding to interfaces instead of implementations, I have a generic DataStore interface that specifies a contract for all I/O involving the data store. This interface can be implemented by concrete classes such as RDBMSDataStore, OODBMSDataStore, XMLFileDataStore, and so on.
This works well as long as I keep the contents of the DataStore interface simple - i.e. getThis(), getThose(), saveThat(), updateThis(), etc. But as soon as I require more complicated queries, it breaks down. The XMLFileDataStore class obviously doesn't understand SQL, and the RDBMSDataStore class obviously doesn't understand XPath/XQuery. And OODBMSDataStore understands something entirely different depending on the OODBMS in use.
I could adopt a language-independent object query language, write all my queries in that and then have the concrete classes translate them into their native language, but that's a huge task, if I want to be complete.
Are there standards or best practices for handling this kind of situation in Java? Unfortunately it seems like 99% of the world interprets "database independence" to mean "relational database independence" and ignores the object databases, XML databases, document databases, etc. entirely.
From the way I read the question, this sounds a lot like the semantic that Hibernate brings to the table for Java. It even has mode for dealing with XML as the content backing store (using Dom4J). The Hibernate API has a number of extension points that could allow the addition of an OODBMS model. Even if Hibernate turns out not to be the best solution for you (implementation-wise), I think it provides a good example of the types of patterns that can be used to solve the problems you proposed.