实体框架 - 从视图创建模型？

Question

I'm brand new to ASPNET and MVC 4, so I imagine that this may be a easy question. However, I've been unable to properly google an answer. I would simply like to display some sales information - I only need to display it. I do not need to insert, update, or delete from the underlying tables.

There are 3 SQL Server tables from which data needs to be pulled: CurrentSales, SalesPlans, and AverageSales. I created a VIEW for this and put a unique clustered index on it; it contains some outer joins but has logic to handle any unlikely NULL values.

I go to MODELS, add a new ADO.NET Entity Data Model, and add my view to the model. It comes back and says "The table/view 'vw_FullView' does not have a primary key defined. The key has been inferred and the definition was created as a read-only table/view." When I create a Controller class from this model and entity, the view displays no data when I load the website.

However, if I create a completely blank table - with an appropriate primary key - that acts as the model table, then use a stored procedure (as a Function Import) to retrieve the data I need, everything works completely fine.

This obviously can't be the correct way to handle this. Is there a way to create a strongly typed model out of a view? I'd prefer to have the Controller and View objects in ASP.NET properly auto-generate from the SQL view in the model, rather than needing this blank table to "fool" the system.

Many thanks in advance.

Answer 1

Entity framework does not like views very much. It cannot figure out what the primary keys are for a view, so it assumes any non-nullable field is part of a composite primary key.

In general, avoid views when using Entity Framework. Create a linq query that does what your view does and use that instead.

Answer 2

a view is just a projection of an entity or a combination of entities. by Default entity Framework will only allow read-only-access as there are no real entities in a view, meaning ET can not track changes.

simmdan actually did a good Job explaining how to work around this in the MSDN forum

basically, as Mystere Man already pointed out, if you use Entity Framework, the easiest way would be to clone your view using the real entities joined and filtered by linq.

Answer 3

It's true that Entity Framework does not like SQL Server views because the primary key is not obvious in a view. Unfortunately, specifying [Key] in the model doesn't seem to be sufficient. However, it can be done and is very useful in the 98% of cases where there's a view that combines tables and you just want to display it's data in a grid. The key (pardon the pun) is to define the view correctly.

My original view contained the following columns:

DepartmentCode(varchar(8),not null)
DepartmentName(varchar(60), not null)
DivisionCode(varchar(8), null)
DivisionName(varchar(8), null)
StatusCode(char(1), not null)
Virtual(varchar(1), not null)

and in my model I specified

[Key]
[Column("DepartmentCode")]
[DatabaseGenerated(DatabaseGeneratedOption.None)]
[StringLength(8)]
[Display(Name = "Department Code")]
public string DepartmentCode { get; set; }

When I scaffolded this and displayed the Index view, I got an error referring to a primary key. Apparently, Entity Framework assumes all the not null fields contribute to a primary key.

Removing the extra not nulls did the trick. I forced the columns in the view to be nullable using the following:

CREATE view [dbo].[Departments] as
select DepartmentCode
,nullif(DepartmentName,'') as DepartmentName
,nullif(DivisionCode,'') as DivisionCode
,nullif(DivisionName,'') as DivisionName
,nullif(StatusCode,'') as StatusCode
,nullif(Virtual,'') as Virtual
from ....

Now the view columns look like this

DepartmentCode(varchar(8),not null)
DepartmentName(varchar(60), null)
DivisionCode(varchar(8), null)
DivisionName(varchar(8), null)
StatusCode(char(1), null)
Virtual(varchar(1), null)

Once I changed the view, the Index view worked correctly.

I also confirmed that a view that contains a compound (multi-column) key will also work as long as only those columns are not null and in your model you specify [Key] on each column and also add Order=1 and Order=2 to the [Column] annotations on those key columns.

Of course, the assumption here is that you have permission to modify the view (or can create an alternate view). I can only confirm this works on MVC 5 and Entity Framework 6.