In my database I have the following tables:
Customers (ID)
Orders (ID)
CustomersOrders (CustomerID, OrderID)
How do I map Customers table to Customers class and Orders table to Orders class without creating a class CustomersOrders?
That depends on which LINQ version you're talking about.
If you're using Entity Framework 4.0 and you have no additional information in the table other than the IDs then what you are asking for should already be generated. I believe the same is true for Entity Framework 1.0.
LINQ to SQL is another story. It never handled Many-to-Many relationships well. You have to allow LINQ to SQL generate the third table and then extend the partial classes by hand in a separate file to mask away that third table. It's ugly but it works. Here's a series of blog posts that detail exactly what needs to be done:
How to implement a many-to-many relationship using Linq to SQL
Related
I have a database with two tables, customer and account. Because multiple customers can exist on multiple accounts this is a many to many design.
This is how I designed it in SQL
This works quite nicely as Entity Framework picks up that its a mapping table and just maps Customer to Account as lists on each and hiding the mapping table. Brilliant!
I would like to extend this further to add preferences to a mapping between a customer and an account like such:
Am I right in assuming this is not possible? I have tried adding it to EF model but instead it brings back the mapping table.
Anyone else had any luck with this?
What you have in your first case is a simple many to many relation table, which in Entity Framework results in the collections of entities on one another.
If you want to have a relation to the Preference from your CustomerAccount relation table, the relation becomes complex and it cannot be depicted in the simple relation lists anymore. You need add an entity for your relation CustomerAccount which will have foreign keys on Customer, Account and Preference.
I'm working on a C# WPF app with a MySQL backend using Entity Data Model in VS2010 (.NET 4). I have the following simple test database schema (ignore StudentCourse.Grade for now):
Students are assigned to courses with a many-to-many relationship via a connecting table. In EDM this is represented as two entities linked by an association mapped to the connecting table:
So far so good.
Now let's say that each student gets a final grade for each course he/she takes which I've stored in the StudentCourse table as Grade. My problems is somehow getting the grade in the EDM. I've tried creating a new StudentCourse entity mapped to the StudentCourse table but it's set to read-only because it has no id. Adding an id to the table causes Visual Studio to whine because the id field is not mapped in the association(which I don't understand at all).
How is a relationship like this mapped in EDM? I'm open to changing the database schema if need be.
Edit in reply to Ucodia:
I don't know. As far as UI goes, grades should be easily accessible for a student or for a course. I'm open to suggestions.
You need to add Grade to the link table and delete the many-to-many relationship. Then regenerate the model from the database. You will find that the StudentCourse table will show in the EDM. When there are additional fields in the link table, the link table will appear in the model.
Why would you not just store the Grade on the Course entity?
Apparently your primary concern is to have something like the Model you designed in EDM designer. If only the object model matters to you, whatever the database looks like, then I would suggest you having a look at EF Code First.
Have a look at this article: EF Code First Walkthrough
By default EF hides a many to many join table that does not contain additional data than the foreign keys to the joined tables.
Is it possible to tell EF (and the designer) to explicitly create the join table and make it usable in code?
Thanks
No EF designer will not add this entity for you. If you want junction table exposed you must manually delete created relation and add junction table's entity and two one-to-many FK relations. Here are related questions with step by step guide:
How to expose the join table in many to many relation
How to get Entity to Table mapping for many to many relations
I am developing ASP.NET (WebForms) app and using EntityFramework. I was wondering what is the best practice of using entities. Create few entities for whole database or create many entities for specialized purposes ?!
Example case is this:
I have customers table. This table have ForeignKey to customers_addresses and customers_phones tables. I got two cases on this dataset
Autocomplete customer name
Show user details
For case 1 I got one entity which have only the "name" column mapped to user to db
For case 2 I got another entity which have all data and connections between other tables.
Now I was wondering if only single entity (number 2) would be good for both cases.
What's the best practice with EF ?
I don't see a reason, in your case, to have a specialized entity for the autocomplete scenario. Since you'll be using Linq to Entities for all your querying, just make sure you are selecting just the Customer Name from the entity, instead of the entire entity itself
from Customer cust in ent.Customers
select new {
CustomerName = cust.CustomerName
};
That way you still have lightweight SQL query on the backend, but you're not polluting your models with unnecessary, "specialized" entities.
In addition, if you're using lazy loading then you don't have to worry about EF loading any related entity information unless you actually need it.
I have a database that relies on a bunch of Many-to-Many and rather than denormalizing the whole database (which is quite large) I would like to know if there is a SIMPLE way of implementing Many-to-Many Relationships using LINQ-to-SQL Yet...
Everywhere I look I find articles on HOW Difficult It is, or A Simple Workaround that is actually "NOT" Simple. But all the articles I have been able to locate are pretty dated, using asp.net 3.5.
Should I just Create a Stored Procedure and then Drag it into the .DBML file as a method?
So I guess my question is:
Is there any new way to implement many-to-many using Linq-to-Sql in ASP.NET 4.0 along with MVC 3.
Examples of usage in my Project:
In the finished model, there will be Other Tables linked to the Many-to-Many Tables (i.e. EventAddress, EventUrl, VipAddress, VipPicture, GuestAddress, GuestPhone, GuestEmail, etc...)
So you can see when this would be a pain in the a$$ to denormalize.
Thank you,
Tim
Have you considered using Entity Framework?
It handles m:n relationships nicely. In your case the EmployeeAddress table would be abstracted away as a list of addresses on your Employee object.
It is also my impression that linq to sql isn't really a priority for Microsoft, EF Is.
I understand if you do not wish to convert an existing project, but for new projects EF4 would probably be a better fit.
LINQ to SQL will always map directly to the structure of the database. If your database has all of those junction tables, then they will be in your LINQ to SQL model - period.
Although you can map many-to-many relationships in both LINQ to SQL
and LINQ to Entities, LINQ to Entities allows a direct mapping of
many-to-many relationships with no intermediary class, while LINQ to
SQL requires that an intermediary class map one-to-many to each of the
classes that are party to the many-to-many relationship.
link to the quote above - http://msdn.microsoft.com/en-us/library/cc161164.aspx