I am working on a database first approach to a web api and I unfortunately have self-referencing loops all over the place. I was wondering if there was a way to set the maximum depth when using entity framework to return a context.
For instance, if I had an Employee and Project relation where the Employee has a Projects field and the Project class has an employee field. I'd like to set a depth of (1) lets say that would return me the employees project objects but inside those project fields, the employee fields can be left blank.
If I run as it goes now, my browser stops everything when I hit a maximum depth of 32.
Related
Problem: When adding an object "Order" to my dbcontext, all nested objects of the order gets "readded" to the database, though the nested objects is static data and only a reference shoudl be added in the database.
Example:
The database holds 0 orders, and 3 items.
I add one order with 2 items.
Now the database hold 1 order, and 5 items. The two items in the order has been "readded" to the database, even though the items had the right primary keys before db.SaveChanges().
I realize that i may be able to attach the existing items to the dbcontext before saving changes, but is that really the only way to go? Can't EF figure out that to item already exists when the primary key matches an existing item?
Does anyone know if this is different in the new version of EF CodeFirst?
No EF cannot figure if entities are existing one or new one - both Add and Attach commands are graph oriented operations. You call them on one entity in the graph and they traverse all relations (and their relations and so on) and perform the operation for them as well.
You must figure correct state of each entity in the graph for example by using:
dbContext.Orders.Add(newOrder);
foreach(var item in newOrder.Items) {
dbContext.Entry(item).State = EntityState.Unchanged;
}
dbContext.SaveChanges();
You can use the reverse operation by calling Attach(newOrder) and set the order to Added state. The main difference will come with independent associations (for example many-to-many relations). The first approach will correctly add new relation between order and each item whereas second will not unless you manually set each relation to Added state (and changing state for relations is more complex).
I am using Entity Framework with asp.net mvc3 razor. Now I have a table which represents Countries (like India, US) etc. And my requirement is I need to open a pop-up with the flags of all countries which I have in my database. And when user click on one flag I need to show that particular country details first and remaining as line by line in the webgrid(asp.net mvc3 razor)
So I prepared a list of "Countries" by getting all the countries from database. And I prepared another list OrderofCountries by adding Order(property) "1" to the which user had clicked. And from 2 to all the remaining countries. Now I need to join this list i.e "OrderofContries" with the remaining tables (from database as for the requirement). But Entity Framework raises an error:
Unable to create a constant value of type 'Slmg.BusinessObjects.CountriesBO'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.
My idea here is by using order property I can sort the data so that I can get the required data.
Can we join our prepared list with the database table in Entity Framework? How to solve my issue. Can any one please help me to find the solution.
No you cannot join list in your application with database table unless you pull all data from that table to your application by calling ToList or AsEnumerable.
I'm simply trying to get data from two sql server db tables using ado.net entity framework. My code is:
using (Model.Entities e = new Model.Entities())
{
return e.PAGE.First().CONTROL;
}
The database is setup to have two tables, a control table which links to a page table via an 'id' field in the tables (control_id). There is one CONTROL object for each PAGE object.
I keep getting a null value for my return value and I know that's not right.
I can use vis studio and breakpoints to see that there is a PAGE object in 'e' and I can see that there are multiple CONTROL objects in 'e'. This isn't a large database, I just have some sample data in there to ensure that I get this working - so I know that there should be a CONTROL object connected to this PAGE (i've verified this through sql server).
I am very familiar with the general code syntax, I've been using LINQ for a couple of years; however, I have not done much work at all with the entity framework or ado.net 4.
It seems like if I just pull individual table data then it works fine (i.e. e.PAGE.First() .. or .. e.CONTROL.Where(x=>x.someValue.Equals('someValue') ) but if I try to pull by traversing through the tables then I get nothing back (NULL).
Hope that all makes sense.
Some questions for you:
I assume is a 1..1 between PAGE and CONTROL,
Is there a FK called "ControlID" on PAGE?
Do you have a navigational property called "Control" on your "Page" entity in your EDMX?
If the answer to all of the above is Yes, then this should work:
var page = e.Pages.Include("Control").First();
Here, you are returning the First "Page" record, and eager loading the associated control.
The SQL produced should be something like this:
SELECT p.*, c.*
FROM Page p
INNER JOIN Control c
on p.ControlId = c.ControlId
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'm building an ASP.NET MVC site using the ADO.NET Entity Framework.
I have an entity model that includes these entities, associated by foreign keys:
Report(ID, Date, Heading, Report_Type_ID, etc.)
SubReport(ID, ReportText, etc.) - one-to-one relationship with Report.
ReportSource(ID, Name, Description) - one-to-many relationship with Sub_Report.
ReportSourceType(ID, Name, Description) - one-to-many relationship with ReportSource.
Contact (ID, Name, Address, etc.) - one-to-one relationship with Report_Source.
There is a Create.aspx page for each type of SubReport. The post event method returns a new Sub_Report entity.
Before, in my post method, I followed this process:
Set the properties for a new Report entity from the page's fields.
Set the SubReport entity's specific properties from the page's fields.
Set the SubReport entity's Report to the new Report entity created in 1.
Given an ID provided by the page, look up the ReportSource and set the Sub_Report entity's ReportSource to the found entity.
SaveChanges.
This workflow succeeded just fine for a couple of weeks. Then last week something changed and it doesn't work any more. Now instead of the save operation, I get this Exception:
UpdateException: "Entities in 'DIR2_5Entities.ReportSourceSet'
participate in the 'FK_ReportSources_ReportSourceTypes' relationship.
0 related 'ReportSourceTypes' were found. 1 'Report_Source_Types' is expected."
The debug visualizer shows the following:
The SubReport's ReportSource is set and loaded, and all of its properties are correct.
The Report_Source has a valid ReportSourceType entity attached.
In SQL Profiler the prepared SQL statement looks OK. Can anybody point me to what obvious thing I'm missing?
TIA
Notes:
The Report and SubReport are always new entities in this case.
The Report entity contains properties common to many types of reports and is used for generic queries. SubReports are specific reports with extra parameters varying by type. There is actually a different entity set for each type of SubReport, but this question applies to all of them, so I use SubReport as a simplified example.
I realise I'm late to this, but I had a similar problem and I hacked through it for about 3 hours before I came up with a solution. I'd post code, but it's at home - I can do it later if someone needs it.
Here are some things to check:
Set a breakpoint on the SaveChanges() call and examine the object context in depth. You should see a list of additions and changes to the context. When I first looked, I found that it was trying to add all my related objects rather than just point to them. In your case, the context might be trying to add a new Report_Source_Type.
Related to the previous point, but if you're retrieving the report source, make sure it is being retrieved from the database by its entity key and properly attached to the context. If not, your context might believe it to be a new item and therefore its required relationships won't be set.
From memory, I retrieved my references using the context.GetObjectByKey method, and then explicitly attached those objects to the context using the context.Attach method before assigning them to the properties of my original object.
I got this error because the table didn't have a primary key, it had a FK reference, but no PK.
After adding a PK and updating the model all is well.
Check if your ReportSource was loaded with the NoTracking option or if its EntityState == 'Detached'. If so, that is your problem, it must be loaded in the context.
This tends to happen if your database tables have a 1 - 1 relationship with each other. In your example reportsourceset expects a reportsorttypes with whatever id it is referencing. I have run into this problem when my relationship is linking two primary keys from opposite tables together.
I've got the same error because of new object instance which created "behind the scene" in "Added" state. This was not obvious.
I got this error when I added the new entity to the context but forgot to add the new entity to its parent's collection in the object graph.
For example:
Pet pet = new Pet();
context.Pets.Add(pet);
// forgot this: petOwner.Pets.Add(pet);