Everything I find through Google refers to Code First so I'm wondering what the Key attribute is actually doing in regards to a Database First design? I'm mainly curious because a lot of the entities contain composite keys so I've been adding the Key annotation to the respective properties, but is this really necessary? And if so, what do I gain from it?
If you're using the database-first workflow (i.e. you have an .edmx file in your solution). Then the Key attribute will have no effect.
However, if you're using the code-first workflow to map to an existing database, the Key attribute tells code first that that property is part of the entity's primary key.
For more info on the different workflows see this video: Visual Studio Toolbox: Entity Framework Part 1
Related
So I'm creating a database model using Entity Framework's Code First paradigm and I'm trying to create two tables (Players and Teams) that must share a uniqueness constraint regarding their primary key.
For example, I have 3 Players with Ids "1", "2" and "3" and when I try to create a Team with Id "2", the system should validate uniqueness and fail because there already exists a Player with Id "2".
Is this possible with data annotations? Both these entities share a common Interface called IParticipant if that helps!
Txs in advance lads!
The scenario you are describing here isn't really ideal. This isn't really a restriction on Entity Framework; it's more a restriction on the database stack. By default, the Id primary key is an Identity column, and SQL itself isn't really supportive of the idea of "shared" Identity columns. You can disable Identity and manage the Id properties yourself, but then Entity Framework cannot automatically build navigation properties for your entities.
The best option here is to use one single participant table, in a technique called "Table Per Hierarchy", or TPH. Entity Framework can manage the single table using an internal discriminator column. Shared properties can be put into the base class, and non-shared properties can be put on the individual classes, which Entity Framework will composite into a single large table in the DB. The main drawback to this strategy is that columns for non-shared properties will automatically be nullable in the database. This article describes this scenario very well.
The more I try to come up with a solution, I realize that this is an example of the XY Problem. There is not really a good solution to this question, because this question is already a proposed solution. There is a problem here that has led you to create an Interface which you suggest requires the entities which are using the interface to have a unique Id. This really sounds like an issue with the design of the Interface itself, as Interfaces should be agnostic to the entity they are applied to. Perhaps providing some code and showing what your problem actually is would be helpful, since the proposed solution you are asking how to implement here isn't really practical.
I did work on ADO.NET Entity Framework v2 about two years ago. I have faint memories.
Incidentally, I happen to be working in a very secured (for want of a euphemism) environment where a lot of links are blocked and there's not much one can do. It does get in the way of studying and working, more than a bit.
Therefore, I have to rely on this forum for a few basic questions I have to get started again. This time, I am working on Entity Framework 4. Here are my questions.
All these questions relate to the EDM generated entities, i.e. not the Code First model.
1) Is my understanding correct? I can rename any column name in the EDM designer generated model and nothing breaks.
2) Foreign keys are expressed as navigational properties in the generated entity classes. No special consideration is required to maintain foreign key relationships. I recall in version 1 of EF, you had just ID properties and the navigational IQueryable/IEnumerable/EntityCollection/RelatedEnd properties were introduced in version 2. I just need to know if I need to do anything to retain/maintain foreign key relationships and referential data integrity. I assume I don't need to. A confirmation would be nice.
3) What are all the possible ways to execute a stored procedure? I recall -- one way to use ExecuteSQL or something on the Context.Database object, another to use EntityClient API and the third was to specify stored procedure names in the InsertCommand, SelectCommand, etc. thingies in the mapping window of the EDM designer. Is this correct?
4) How do you use those SelectCommand thingies in the mapping window on an entity? Do you just supply the names of the stored procedures or user defined SQL functions?
5) How do I create an entity out of a subset of a table? Do I just create an entity from the table and then delete the columns I don't need from the designer? What happens if there are required (NOT NULL in SQL) values that are not in the subset I choose?
6) How do I create a table out of a query or join of two or more tables.
1) You can rename columns in the designer and nothing should break (if the name is valid)
2) Navigation properties point to related entities. In EF4 foreign keys were added. Foreign key properties basically expose the database way of handling relations. However, they are helpful since you don't have to load related entities just to change relationship - you can just change the key value to the id of the other entity and save your changes
3) Yes. You can either execute the procedure directly - this is for stored procedures that are not related to CUD (Create/Update/Delete) operations. You can map CUD operations to stored procedure as opposed to having EF execute SQL statements it came up with for your CUD operations.
4) I think this is a bit out of scope - you probably can find a lot of blogs with images how to do it. Any decent book on EF should also have a chapter (or two) on this. Post a question to stackoverflow if you hit a specific problem.
5) Remove properties in the designer and then supply default value to the S-Space definition. I believe you cannot do this with the designer. You need to rightclick on the edmx file and open it with the Xml editor and manually edit it. Also see this: Issue in EF, mapping fragment,no default value and is not nullable
6) You can either create a view in the database or you can create entity set using E-SQL in your edmx file (I think this will be read only though) or you may be able to use Entity Splitting. Each of these is probably a big topic itself so I think you should find more about these and come back if you have more specific questions / problems
I'm planning on using the Entity Framework 4.1 in my next project, but I'm having trouble finding a good way to go about it.
In short, I want to build a multi-tiered application in which the entities will be travelling through web services, and to keep it all as clean as possible I want to use POCO's rather than self tracking entities. Also, there already exists a SQL 2008 database that will be used to base the entities on.
From what I've read so far (from Julie Lerman's article on http://msdn.microsoft.com/nl-nl/magazine/hh148150%28en-us%29.aspx, amongst others), it seems that:
If you use the Database First approach, you get a beautiful .edmx to edit your model in, but you'll always end up with persistence-aware objects rather than POCO's, which is not useful in my situation.
If you use the Code First approach, the "ADO.NET DbContext generator" only partially helps you: it does generate entities from the .edmx, but it doesn't generate the code required to get the foreign keys and cardinality correct. This means that the code will not work out-of-the-box (-edit, not true, see my post below-), you either have to
a) use Data Annotations on your POCO's, which is ugly imo because it pollutes the POCO's with database information and also creates a dependency on the EntityFramework assembly.
b) use the DbModelBuilder passed to DbContext.OnModelCreating to set the correct foreign key, mapping etc. information (i.e. the 'fluent' API). And even though the API may be 'fluent', it's still pretty hard (and probably unmaintainable) to set all this information correctly so that it matches the existing database (see http://sessionfactory.blogspot.com/2011/04/conventions-in-entity-framework-41.html for some examples of this).
I realize that the reason why the "DbModelBuilder-way" requires so much effort is because it was designed to be used the other way around: you're supposed to generate the database from the Entity definitions, not try to tweak all Entities so that they (hopefully) match an already existing database. However, it seems to me that the "DbModelBuilder-way" will, in the end, produce the best result: pure POCO's with no database metadata in them.
Now, having said all this, my question is:
Does anyone know of a way to load an .edmx into the DbModelBuilder, so that the foreign key, column mapping and other information doens't need to be specified by hand through the fluent API?
I think that this would be the best of both worlds, because you can visually edit the mapping like you would in the Database First scenario, and still get clean POCO's because all required metadata is stored in the DbModelBuilder.
Man what are you talking about?
If you use the Database First approach, you get a beautiful .edmx to
edit your model in, but you'll always end up with persistence-aware
objects rather than POCO's, which is not useful in my situation.
That is not true. EDMX can produce almost everything code first can and even many things which code first can't.
If you use the Code First approach, the "ADO.NET DbContext generator"
only partially helps you: it does generate entities from the .edmx,
but it doesn't generate the code required to get the foreign keys and
cardinality correct. This means that the code will not work
out-of-the-box, ...
That is not true. Once you set up EDMX correctly it will create exactly entities you want.
Does anyone know of a way to load an .edmx into the DbModelBuilder
That way is DbContext T4 generator!
Anyway there is one more tool you can check: EF Power Tools CTP1. This tool can create code first mapping from existing database.
Of course every tool creates model which is 1:1 mapping to the database. If you want anything more you must modify the model or mapping manually!
Apparently, there are two ways you can use the generated code from the "ADO.NET DbContext Generator", depending on which type of connection string you use.
If you use an entity connection string, i.e.:
<connectionStrings>
<add name="MyDBEntities" connectionString="metadata=res://*/Model1.csdl|res://*/Model1.ssdl|res://*/Model1.msl;provider=System.Data.SqlClient;provider connection string="Data Source=.;Initial Catalog=MyDB;Integrated Security=True;MultipleActiveResultSets=True"" providerName="System.Data.EntityClient" />
</connectionStrings>
the DbContext will be filled with all the metadata (column mappings, foreign key relations, etc.) from the .edmx. Also, the DbContext's OnModelCreating isn't executed. This is Database First using POCO's, and this is what I wanted to achieve.
What I did wrong was that I used a regular SQL connection string to pass into the DbContext. This causes a totally different type of behaviour: because the DbContext is now empty, it will try to explore all Entity classes and use conventions to generate a database schema for this. Now the OnModelCreating is called, expecting you to tweak this mapping and then generate a database from this.
In short, the solution was to use an entity connection string rather than a SQL connection string.
Read about the T4 POCO generation templates:
Walkthrough: POCO Template for the Entity Framework
POCO Template Code Generation Options
Using the POCO T4 Code Generation Template for Entity Data Models (video screencast with Julie Lerman)
Those T4 code generation templates allow you to use
database-first approach with EDMX model file
but: generate simple POCO (plain-old CLR objects) which have no inheritance from any EF specific class - they're just absolutely plain CLR classes.....
This gives you the best of both worlds - a nice EDMX model and persistence-ignorant POCO classes.
I see some posts on programmatically mapping a table with a composite key but I haven't found any examples to do that within xml of the edmx file. How do I do this? In the edmx of my entity type I have a key section with multiple propertyref's pointing to my composite key. However, when I go to save my changes it errors and states that there is no update function.
It seems to me, if I mark one key as the primary key and it can manage that it should be able to use a composite key as well.
Please do not reply and state that I shouldn't be using composite keys because I don't care and have no ability to change it. I didn't develop this database as it is a third-party ERP application that I'm interfacing with.
From what I can tell I may have to create a stored procedure just to save it? Isn't there a better way?
Thank You.
I'm working on a C#.NET 3.5 WCF RIA Services app and having an issue with my Entity Framework model.
My entity Foo is mapped to a DB table and has a primary key called FooId. My Bar is mapped to a DB view. I've selectively designed this view to generate a composite key in the EF using two of the columns (by making sure they were non-nullable and the others are all nullable. This was done using NULLIF and ISNULL in the view design.)
I'm able to add this view to the model with no problem but I keep running into an issue when I try to map an association between the two. Foo should contain many Bars but I keep getting the following error when I add the association:
Unable to retrieve AssociationType for
association 'FK_Bar_Foo'
According to this page, it looks like this might work if I can properly name the association (since RIA Services looks for specific names.) I've tried several variants of names that match the pattern of other associations with no success. Does anyone know if there's a place I can look to find out what name it's looking for?
Thanks,
After some research I found a workaround to the issue here. The problem with this solution is that you have to repeat it every time the model is updated, which just won't work for me. It appears the current version of the EF doesn't support this type of relation.
The solution I finally went with was to redesign the client to independently call the service and request an entity collection by passing the primary key (FooId) of my Foo type. It's not the best approach (and requires a lot more manual coding) but it does the job. I hope Entity Framework version 4 solves this limitation.