I have a Cars table, a BodyPaints table, and a Cars_BodyPaints association table that defines what paint jobs are possible for various cars. The association between Cars and BodyPaints is many to many.
I'd like to create an extra piece of data for each (Car, Paint) mapping to define the cost of a specific paint job on a specific car. I have added an extra column to Cars_BodyPaints to record the price of each pairing, but when I update the model classes from the DB I don't see it exposed in any of the entity classes. This leads me to believe that maybe my approach is wrong here. I was excepting some method to be generated so I could execute code like:
Car civic = (from c in context.Cars where c.Name == "Civic" select c).Single();
Paint red = (from p in civic.Paints where p.Name == "Red" select p).Single();
var price = civic.GetPrice(red);
Am I off base here? How would you accomplish this?
Thanks
When modeling many-to-many relationships with attributes, you will need to model the relation as an entity itself. You would create a new entity for the Cars_BodyPaints table and it would contain your extra price column as a property. This new intermediary entity must be traversed when considering the relation between Cars and BodyPaints and can also be directly queried.
Cars <-- Cars_BodyPaints --> BodyPaints
In this new model, your query would look more like:
(from bp in context.CarBodyPaints
where bp.Car.Name = "Civic" and bp.Paint.Name = "Red"
select bp.Price).Single()
Related
Is it possible to include a computed field in an EF entity? Example, lets say I have a shop selling products, and the products are grouped into categories.
When the list of categories is displayed for editing to the administrator of the shop I wish to list the number of products in each category as part of the list.
In NHibernate I would use a formula e.g.
<property name="Products" formula="dbo.Category_NumProducts(id)" />
I can't work out if something similar is possible for EF. I know I could make a category view and map this to a second entity but that seems wrong as its almost a complete duplication.
EDIT: If this isn't possible in EF, then what is the best way to accomplish this?
Unfortunately if your Category table doesn't have this as computed column you will not be able to map it without creating second entity - it leads to database view, defining query in EDMX or perhaps Query view in EDMX (but Query view may not work because you require aggregate function to be computed).
Edit:
IMHO the simplest solution (and also the best) is simply creating a ViewModel (some non mapped class) and use projection in Linq query:
var query = from c in context.Categories
where ...
select new CategoryView {
Id = c.Id,
Name = c.Name,
...
Products = c.Products.Count()
};
so the story is very simple.
I have one table called Products and another Called categories. In addition, i have another table called ProductCategories that hold the relationship of catetories to their corresponding products (i.e, the table has two columns, ProductId, ColumnId).
For some reason, after adding all those table to my entity model, i don't have "Access" to it, hence i can do myentityModel.ProductCategories, so i could relational items between those two tables.
And yes, the ProductCategores table is added as "Association" to the entity model. i don't really understand that.
EDIT:
I do see that as part of creating new "Product" i can pass EntityCollection of "Category". So i do query from my entity model for a list of the matching categories that the user selected (on the webpage). so for example, i get (after query the model), an Objectset of "Category". However, i encountered two issues:
the 'AddObject' accept only EntityCollection, hence i need to re-create a set and then add all the objects from the ObjectSet to the entityCollection, in this process i need to detach it from the previous model and add it to the new collection. if not, i get an exception.
when i do the SaveChanges, i see that i get an exception that it was actually trying to Create new Category rather than adding new ProductCategory. again, am i missing something here?
Thanks.
This sounds like a Many-to-Many relationship. In your entity model, you don't need to declare the join table as a separate entity. Instead, you configure the relationship between the Products and the Categories as a Many-to-Many and add metadata about the join table. In Hibernate, you would have:
#ManyToMany(targetEntity=Categories.class, cascade={CascadeType.ALL}, fetch = FetchType.LAZY)
#JoinTable(name="tb_products_categories",
joinColumns=#JoinColumn(name="category_id"),
inverseJoinColumns=#JoinColumn(name="product_id")
)
#IndexColumn(name="join_id")
public List<Categories> getCategories() {
return categories;
}
When you query, the ORM layer takes care of determining SQL and traversing table joins.
I have an entity that has a relationship with another entity. I am able to search on columns that are in the main entity, and include columns from the relationship entity. But I need to be able to filter the list (search) on columns that are not in the relationship entity.
for example
the Invoice Entity contains a customerId property, and is related to the Customer Entity which contains the customerName property
I need to be able to search / filter the grid by customerName.
I am new to entity framework, please help.
thanks
Carl
Your relation is 1->1. In these cases I usually return a custom class to the grid that has all the columns I need, including joins with other tables.
So basically what you need is to create a custom linq query with your resultset.
The mais query should follow this example:
var q = from i in ctx.Invoices
join c in ctx.Customers on i.CustomerID equals c.CustomerID
select new{InvoiceID=i.InvoiceID, InvoiceDate=i.Date, CustomerName=c.Name};
Now, assuming we receive a CustomerName variable with the string to filter by c.Name we could do:
if(!string.IsNullOrEmpty(CustomerName))
{
q = q.where(c => c.Name.ToLower().Contains(CustomerName.ToLower()));
}
Notice that I'm performing a ToLower() operation and a Contains, this will beahave as a LIKE ingnoring case sensitivity and searching for the string anywhere in the Customer Name.
At the end you'll return the q.ToList() serialized for the jqGrid...
Please help an EF n00b design his database.
I have several companies that produce several products, so there's a many-to-many relationship between companies and products. I have an intermediate table, Company_Product, that relates them.
Each company/product combination has a unique SKU. For example Acme widgets have SKU 123, but Omega widgets have SKU 456. I added the SKU as a field in the Company_Product intermediate table.
EF generated a model with a 1:* relationship between the company and Company_Product tables, and a 1:* relationship between the product and Company_Product tables. I really want a : relationship between company and product. But, most importantly, there's no way to access the SKU directly from the model.
Do I need to put the SKU in its own table and write a join, or is there a better way?
I just tested this in a new VS2010 project (EFv4) to be sure, and here's what I found:
When your associative table in the middle (Company_Product) has ONLY the 2 foreign keys to the other tables (CompanyID and ProductID), then adding all 3 tables to the designer ends up modeling the many to many relationship. It doesn't even generate a class for the Company_Product table. Each Company has a Products collection, and each Product has a Companies collection.
However, if your associative table (Company_Product) has other fields (such as SKU, it's own Primary Key, or other descriptive fields like dates, descriptions, etc), then the EF modeler will create a separate class, and it does what you've already seen.
Having the class in the middle with 1:* relationships out to Company and Product is not a bad thing, and you can still get the data you want with some easy queries.
// Get all products for Company with ID = 1
var q =
from compProd in context.Company_Product
where compProd.CompanyID == 1
select compProd.Product;
True, it's not as easy to just navigate the relationships of the model, when you already have your entity objects loaded, for instance, but that's what a data layer is for. Encapsulate the queries that get the data you want. If you really want to get rid of that middle Company_Product class, and have the many-to-many directly represented in the class model, then you'll have to strip down the Company_Product table to contain only the 2 foreign keys, and get rid of the SKU.
Actually, I shouldn't say you HAVE to do that...you might be able to do some edits in the designer and set it up this way anyway. I'll give it a try and report back.
UPDATE
Keeping the SKU in the Company_Product table (meaning my EF model had 3 classes, not 2; it created the Company_Payload class, with a 1:* to the other 2 tables), I tried to add an association directly between Company and Product. The steps I followed were:
Right click on the Company class in the designer
Add > Association
Set "End" on the left to be Company (it should be already)
Set "End" on the right to Product
Change both multiplicities to "* (Many)"
The navigation properties should be named "Products" and "Companies"
Hit OK.
Right Click on the association in the model > click "Table Mapping"
Under "Add a table or view" select "Company_Product"
Map Company -> ID (on left) to CompanyID (on right)
Map Product -> ID (on left) to ProductID (on right)
But, it doesn't work. It gives this error:
Error 3025: Problem in mapping fragments starting at line 175:Must specify mapping for all key properties (Company_Product.SKU) of table Company_Product.
So that particular association is invalid, because it uses Company_Product as the table, but doesn't map the SKU field to anything.
Also, while I was researching this, I came across this "Best Practice" tidbit from the book Entity Framework 4.0 Recipies (note that for an association table with extra fields, besides to 2 FKs, they refer to the extra fields as the "payload". In your case, SKU is the payload in Company_Product).
Best Practice
Unfortunately, a project
that starts out with several,
payload-free, many-to-many
relationships often ends up with
several, payload-rich, many-to-many
relationships. Refactoring a model,
especially late in the development
cycle, to accommodate payloads in the
many-to-many relationships can be
tedious. Not only are additional
entities introduced, but the queries
and navigation patterns through the
relationships change as well. Some
developers argue that every
many-to-many relationship should start
off with some payload, typically a
synthetic key, so the inevitable
addition of more payload has
significantly less impact on the
project.
So here's the best practice.
If you have a payload-free,
many-to-many relationship and you
think there is some chance that it may
change over time to include a payload,
start with an extra identity column in
the link table. When you import the
tables into your model, you will get
two one-to-many relationships, which
means the code you write and the model
you have will be ready for any number
of additional payload columns that
come along as the project matures. The
cost of an additional integer identity
column is usually a pretty small price
to pay to keep the model more
flexible.
(From Chapter 2. Entity Data Modeling Fundamentals, 2.4. Modeling a Many-to-Many Relationship with a Payload)
Sounds like good advice. Especially since you already have a payload (SKU).
I would just like to add the following to Samuel's answer:
If you want to directly query from one side of a many-to-many relationship (with payload) to the other, you can use the following code (using the same example):
Company c = context.Companies.First();
IQueryable<Product> products = c.Company_Products.Select(cp => cp.Product);
The products variable would then be all Product records associated with the Company c record. If you would like to include the SKU for each of the products, you could use an anonymous class like so:
var productsWithSKU = c.Company_Products.Select(cp => new {
ProductID = cp.Product.ID,
Name = cp.Product.Name,
Price = cp.Product.Price,
SKU = cp.SKU
});
foreach (var
You can encapsulate the first query in a read-only property for simplicity like so:
public partial class Company
{
public property IQueryable<Product> Products
{
get { return Company_Products.Select(cp => cp.Product); }
}
}
You can't do that with the query that includes the SKU because you can't return anonymous types. You would have to have a definite class, which would typically be done by either adding a non-mapped property to the Product class or creating another class that inherits from Product that would add an SKU property. If you use an inherited class though, you will not be able to make changes to it and have it managed by EF - it would only be useful for display purposes.
Cheers. :)
I have 3 entities
-Direction
-City
-GeoPosition
each Direction have a Geoposition, and each City have a collection of Geopositions (this represent a polygon)
I have 5 tables
-directions
-cities
-geopositions
-directionsgeopositions
-citiesgeopositions
and EF entities is this
alt text http://img192.imageshack.us/img192/5863/entitydesignerdiagram.png
each entity have function imports for insert, update, and delete
i have this error
Error 2027: If an EntitySet or AssociationSet includes a function mapping,
all related entity and AssociationSets in the EntityContainer must also define
function mappings. The following sets require function mappings: CitiesGeopositions, DepartmentsGeopositions.
I need function imports for the relation tables??
what is the problem?
The answer to your questions are, respectively:
Yes.
See (1).
The Entity Framework allows you to insert/update/delete via DML or stored procs, but it does not allow you to choose "both." If you are going to go to the stored proc route, you must supply procs for every sort of data modification the framework might need to do on an entity, including relation tables.
For a couple of days now, I have been wracking my brains and scouring the Interwebz for information about how to insert data into database intersection tables using the Entity Framework (EF). I’ve hit all the major players’ web sites and blogs and NO ONE has provided straightforward syntax on how to perform this. Out of the blue, the answer occurred to me and I was bound and determined to share this with as many people as I could to lessen the pain I went through.
Let’s set the stage. Assume we have a database relationship as such:
Students (StudentID(PK), StudentName, Gender)
Courses (CourseID(PK), CourseName, CourseDescription)
StudentsCourses (StudentID(PK, FK), CourseID(PK, FK))
For those of you familiar enough with EF, you know that when the relationships above are translated into an entity data model, the Students and Courses tables are created as entities, but the StudentsCourses table is not. This is because the StudentsCourses table does not contain any attributes other than the keys from the other two tables, so EF directly maps the many-to-many relationship between Students and Courses (EF is not limited in the way relational databases are in this respect.) and instead of an entity, translates the intersection table into an AssociationSet. If you weren’t aware of this behavior, check out these links for examples:
http://thedatafarm.com/blog/data-access/inserting-many-to-many-relationships-in-ef-with-or-without-a-join-entity/
http://weblogs.asp.net/zeeshanhirani/archive/2008/08/21/many-to-many-mappings-in-entity-framework.aspx
Now let’s assume that you want to register a current student (ID:123456) for new courses this semester (ENGL101, SOC102, and PHY100). In this case, we want to insert new records into the StudentsCourses table using existing information in the Students table and Courses table. Working with data from either of those tables is easy as they are both an entity in the model, however you can’t directly access the StudentsCourses table because it’s not an entity. The key to this dilemma lies with the navigation properties of each entity. The Student entity has a navigation property to the Course entity and vice versa. We’ll use these to create “records of association” as I like to call them.
Here’s the code sample for associating an existing student with existing courses:
using (var context = TheContext())
{
Student st = context.Students.Where(s => s.StudentID == “123456”).First();
st.Courses.Add(context.Courses.Where(c => c.CourseID == “ENGL101”).First());
st.Courses.Add(context.Courses.Where(c => c.CourseID == “SOC102”).First());
st.Courses.Add(context.Courses.Where(c => c.CourseID == “PHY100”).First());
context.Students.AddObject(st);
context.SaveChanges();
}
Because the associations go both ways, it stands to reason that one could retrieve three Course objects (by CourseID) and associate the same Student object to each, but I haven’t tested that myself. I think it would result in more code than is necessary and might be semantically confusing.
Here’s a code sample that associates a new student with the same existing courses:
using (var context = TheContext())
{
Student st = new Student({ StudentID = “654321”, StudentName = “Rudolph Reindeer”,
Gender = “Male” });
st.Courses.Add(context.Courses.Where(c => c.CourseID == “ENGL101”).First());
st.Courses.Add(context.Courses.Where(c => c.CourseID == “SOC102”).First());
st.Courses.Add(context.Courses.Where(c => c.CourseID == “PHY100”).First());
context.Students.AddObject(st);
context.SaveChanges();
}
And finally, here’s the code to associate a new student with new courses (‘...’ used for brevity):
using (var context = TheContext())
{
Student st = new Student({ ... });
st.Courses.Add(new Course({ ... }));
st.Courses.Add(new Course({ ... }));
st.Courses.Add(new Course({ ... }));
context.Students.AddObject(st);
context.SaveChanges();
}