Develop Reference

NHibernate map join table with extra column

I have 3 tables: Workspace with PK IdWorkspace and other columns, Language with PK IdLanguage and other columns and join table WorkspaceFooter with multiple PK (IdWorkspace FK and IdLanguage FK) and extracolumn string Denomination. I cannot find a way to map those in Nhibernate, a way that WORKS.
How should I write in the .hbm.xml files? What should I map in Workspace? A collection, a list of WorkspaceFooter etc.?
Please keep in mind that for some reason the relationship in Workspace that points to WorkspaceFooter needs to be inverse="true". An update in Workspace must force an update in WorkspaceFooter too.
Thank you in advance.

You need to be clear about:
the class model you need (derived from the business logic, not just the database model)
Whether you really need a bidirectional relation
You could:
map it as an independent entity in a list (which is the solution with the most management code required),
map it as a composite element with the denomination and a many-to-one to the language. It also uses a specific class in C# for this. Probably the most straight forward solution.
map it as a map (dictionary in C#), where the denomination is the key (given that it is unique within the same workspace and a typical way access the language),
probably you can map it as a list and use filters to access only data of a certain denomination. I don't have experience with filters and I don't know what kind of troubles you may face.

EF DB-first mapping mess

I have a frustrating situation owing to this little quirk of EF. Here's a simple demo of the behavior. First the DB schema:
As you see, RestrictedProduct is a special case of product, which I'm intending to make a subclass of Product with some special code.
Now I import to an EF data model:
Oops! EF saw that RestrictedProduct had only 2 fields, both FKs, so it mapped it as a one-to-many relationship between Product and Restriction. So I go back to the database and add a Dummy field to RestrictedProduct, and now my EF model looks much better:
But that Dummy field is silly and pointless. Maybe I could delete it? I blow away the field from the DB table and the entity model, then refresh the model from the DB...
Oh, no! The Product-Restriction association is back, under a new name (RestrictedProduct1)! Plus, it won't compile:
Error 3034: Problem in mapping fragments starting at lines (x, y) :Two entities with possibly different keys are mapped to the same row. Ensure these two mapping fragments map both ends of the AssociationSet to the corresponding columns.
Is there any way to prevent this behavior, short of keeping the Dummy field on the RestrictedProduct table?

I just came across the same issue, and as an alternative to putting the dummy field in your RestrictedProduct table to force the creation of an entity you can also make your RestrictedProduct.RestrictionId field nullable and EF will then generate an entity for it. You can then modify it to use inheritance and any subsequent "Update model from database" will not cause undesired nav properties. Not really a nice solution but a work around.

Let's walk slowly into your problem.
1st thing you need to decide is if the restricted product is
really a special case of product or is it a possible extension
to each product.
From your original DB Scheme it seems that any product may have
a relation to a single restriction however a single restriction
can be shared among many products.. so this is a simple 1 to many
situation which means that restricted product is NOT a special case
of product! Restriction is an independent entity which has nothing
to do with product in a specific way.
Therefore EF is correct in the 1st importation of your scheme:
1. a product can have 0 or 1 restrictions.
2. a restriction is another entity which can be related to many products.
I do not see your problem.

Creating a custom entity with the Entity framework

If I have for example two entities, lets say Customers and Staff with no relation between them, is it possible to create a third entity which doesn't have a corresponding table in the database which takes some information from the first and the second entity and also one or two additional columns (for example computed columns)?

You can join entities through this: http://blogs.msdn.com/b/simonince/archive/2009/03/23/mapping-two-tables-to-one-entity-in-the-entity-framework.aspx
But I do believe they have to be related; otherwise, how would it know what information was correctly tied to each other? The only thing it could do is query all rows of one entity and match them up with all rows of the other, which is probably not what's desired.

Entity Framework 4 and SQL Server 2008 Multiple Possible Foreign Keys

I am trying to come up with a database design that would work with Entity Framework 4 Code First. Actually, I have no experience yet of EF4 Code First but as I understand it, if I write the code, it will create the database and tables.
The issue is this. There are various types of auctions, they all have some common fields and some specific ones. In the code I envisage having a base abstract class called Auction and subclasses like LowestUniqueBidAuction and EnglishForwardAuction etc.
Nothing surprising there. The problem is that I imagine the database structure to mimic this. I imagine an Auction table and a LowestUniqueBidAuction table and a EnglishForwardAuction table. In the Auction table I imagine a foreign key into one of these two tables for each row depending on the type of auction that that row is. I also imagine another column in the Auction table with the name of the derived auction table (such as EnglishForwardAuction).
The problem is that whenever I've ever created a foreign key I've had to specify the name of the foreign table into which the key points (which makes sense). In this case, however, there is one of many tables that the key could point. So there are many issues here. Firstly, I could simply not use a foreign key and just use an ordinary field, but then the database will not be able to maintain data consistency for me. The second issue is how will EF Code First handle this? In other words, how will it know that if I ask for all EnglishForwardAuction rows from the Auction table that it should look at the column with the table name and then join on the EnglishForwardAuction table to get the extra fields?
Has anyone ever faced similar issues?
Thanks,
Sachin

This problem is solvable in Entity Framework in a number of ways - read up on how EF handles inheritance and what strategies are available.
There are basically three strategies how to handle this:
(1) Table per Hierarchy
You have only one single table, that represents all possible sub classes. Of course, this means, several rows (that only exist in a given subclass) must be nullable, since they don't show up / don't exist in super classes or other subclasses.
(2) Table per Type
Each subclass gets its own table, and by default, the sub-types table shares the PK with the base classes' table - e.g. PK = 1 in Auction will also be PK = 1 in EnglishForwardAuction. So your subclass tables reference the base table - not the other way around.
(3) Table per Concrete Type
Each concrete subclass (your separate auction types) gets its own table, but that table contains everything - all the columns, from that specific type, but also its base type.
Read more here:
Inheritance in the Entity Framework
Inheritance and Associations with Entity Framework Part 1
Entity Framework Modeling: Table Per Hierarchy Inheritance
Entity Framework Modeling: Table Per Type Inheritance
Searching for Entity Framework Inheritance and/or one of these strategies will reveal a lot more hits, too - that topic is very well covered and discussed on the interwebs! :-)

Entity Framework - making a large edmx table more manageable by splitting?

I think this is a question of the best technique or best way to skin a cat!
Imagine a menu with items (menu choices) on it. I have a table called MenuItem, which for example "Spaghetti Bolognese", it has lots of other information associated with it aside from just a better description and picture.
Eg.
Basic Information (Name, Description, Picture, etc)
Nutritional Information (approx 15 columns)
Allergy Information (approx 16 columns)
Dietary Information (another 7 columns) (religious etc)
As it is at the moment I have it all in the one table in SQL server, which is logical database design to me as it doesn't repeat, despite it making the field list for the table longer than I would like. I'd already been feeling a bit bad about just continually extending the database table. But now we also want to add 'Recipe' information, approx another 7 columns.
I'm using Entity Framework 4.latest, and feel there is probably functionality to help me split this off within the EDMX? (Is that what ComplexTypes are?) Or do I just need to do this in the ViewModel class I call?
I think what I'm after using in my code to segregate things better is something like
MenuItem.Recipe.Ingredients
MenuItem.Nutrition.Fat
etc

Complex types can help you but be aware that complex types cannot contain navigation properties, cannot be null and are always loaded with the entity. Other possibility is to use table splitting - this will allow you to map multiple one-to-one related entities to the same table. The main features of table splitting are:
Entities can share only primary key properties
There is one main entity and others are considered as relations (navigation properties)
Related entities must exists - they are not optional so when you insert new main entity you must insert these related entities as well even if they are empty
Related entities must be loaded with eager, lazy or explicit loading