问题
I wrote a few assumptions regarding Entity Framework, then a few questions (so please correct where I am wrong). I am trying to use POCOs with EF 4.
My assumptions:
- Only one data context can exist for an EF diagram.
- Data Contexts can refer to more than one entity.
- If you have two data sources, say MS SQL server and Oracle, EF requires two different diagrams to access the data.
- The EF diagram data context is the "Unit of Work", having a single Save() for anything on the diagram. (Sure you could wrap it in a UnitOfWork class, but it essentially has the same duties).
Assuming that's correct, here are my questions:
If you don't keep all entities on the same EF diagram, how do you maintain data integrity, like "Orders" cannot exist without a "Customer"? Is this solely a function of the repository to load data just to verify integrity, or do we "try/catch" on database referential integrity errors?
Wouldn't you create an EF diagram for each Entity? For example, I wouldn't expect changes to a customer and changes to a product to be written together as they have nothing to do with each other (having them on the same diagram would cause them to be written together). Or is the scope of an EF diagram to encompass all similar entities stored in the same storage medium?
Is it the norm to divide up the entities like that, or just have a single diagram holding all the entities? I would think the latter, but the thinking is getting the better of me.
回答1:
Having one big EDM containing all the entities generally is NOT a good practice and is not recommended.
Using one large EDM will cause several issues such as:
Performance Issue in Metadata Load Times:
As the size of the schema files increase, the time it takes to parse and create an in-memory model for this metadata would also increase.
Performance Issue in View Generation:
View generation is a process that compiles the declarative mapping provided by the user into client side Entity Sql views that will be used to query and store Entities to the database. The process runs the first time either a query or SaveChanges happens. The performance of view generation step not only depends on the size of your model but also on how interconnected the model is. If two Entities are connected via an inheritance chain or an Association, they are said to be connected. Similarly if two tables are connected via a foreign key, they are connected. As the number of connected Entities and tables in your schemas increase, the view generation cost increases.
Cluttered Designer Surface:
When you generate an Edm model from a big database schema, the designer surface is cluttered with a lot of Entities and it would be hard to make sense of how your Entity model in total looks like. If you don’t have a good overview of the Entity Model, how are you going to customize it?
Intellisense experience is not great:
When you generate an Edm model from a database with say 1000 tables, you will end up with 1000 different entity sets. Imagine how your intellisense experience would be when you type “context.” in the VS code window.
Cluttered CLR Namespaces:
Since a model schema will have a single EDM namespace, the generated code will place the classes in a single namespace.
For a more detailed discussion, have a look at Working With Large Models In Entity Framework – Part 1
Solution:
While there is no out of the box solution for this but it suggests that instead, you should Naturally Disconnected Subsets in your model meaning that based on your domain model, you should come up with different sets of domain models each containing related objects while each set is unrelated and disconnected from the other one. No Foreign Keys in between could be a good sign for separation. And this make sense because in a large model, usually your application does not require all the tables in a database to be mapped to one Entity Model in order to work.
Even if this kind of separation is not 100% possible - meaning that there are subsets of tables that have out going foreign keys to other tables in the database - it still encourages you do separate them. When you do this, you would have to take the responsibility of setting the foreign key appropriately. There would be no navigation property that allows you to get the Entity that represents this foreign key. Of course you could manually query for this Entity in the other container if needed.
Also, for some tips and tricks on how you can split one large entity model into smaller ones while reusing types, take a look at: Working With Large Models In Entity Framework – Part 2
About your question: Order and Customer belong to the same natural domain and should be kept in the same EDM. Like I said, you can scatter them over 2 different entity data models but then you have to take the responsibility of setting the appropriate foreign keys or you'll get runtime exceptions, by the same token, Customer and Product should be kept in separate entity data models. Following these rules, you can come up with a well defined domain set design in your data access layer.
回答2:
I realize that this question was about EF4 but I am sure that many people who are just now "making the switch" will end up here via Google and will read this and the approved answer and make decisions based on it even though they are using EF5 (or EF4.4 if you are stuck on .Net 4.0)
EF5 allows multiple diagrams per edmx. This is a big deal, at least to my team, because it allows us to visually separate entities without requiring separate edmx files. Dr. Zim's points are all still valid except (obviously) the "cluttered designer surface".
There are draw backs to having multiple edmx files, the biggest one is that even if you create separate namespaces for each, you cannot duplicate entity names. Yes, if you truly are designing your system "code first" then this should not be a problem. However, many (most) of us are adding EF to existing systems that are already built on top of relational databases which have normalization.
"But normalization is a good thing, right?" Well, if you are using a relational database yes. "But why does that matter if I am using EF?" A common "normalized" table is Address. Possible scenario: Company (location of business/office) and Contact (might be "remote" worker so they are not at the business location) and they both have a FK that points to Address. Using one edmx file for Company and one for Contact (even with different namespaces) that both include the Address table, the code will compile but at run time you will get this beauty:
Multiple types with the name 'Address' exist in the EdmItemCollection
in different namespaces. Convention based mapping requires unique names
without regard to namespace in the EdmItemCollection
You can change the mapping that is used by EF but then you have other "issues" when working through implementation and most people use the default mapping so forums like this won't have many pertinent questions and answers.
You could also rename the Model name for the Address table to "ContactAddress" and "CompanyAddress" respectively, but that gives the illusion that they are different types when they really aren't. OK, so they are different types in EF but not in the database and, as I said, most of us "live" in the world of tacking on EF to an existing system with an existing data store that is a relational database.
This is already a long-winded "answer" so I will stop here. I just wanted to make sure that people who landed here because they searched for "multiple edmx" and did not realize that there are significant difference between EF4 and EF5 were made aware and realized they may need to do some more investigating.
来源：https://stackoverflow.com/questions/3867479/entity-framework-4-does-it-make-sense-to-create-a-single-diagram-for-all-entiti