Just starting out with Entity Framework (Code First) and I have to say I am having a lot of problems with it when loading SQL data that is fairly complex. For example, let's say I have the following tables which stores which animals belongs to which regions in the world and the animal are also categorized.
Table: Region
Id: integer
Name string
Table AnimalCategory
Id integer
Name: string
RegionId: integer -- Refers back Region
Table Animal
Id integer
AnimalCategoryId integer -- Refers back AnimalCategory
Let's say I want to create a query with Entity Framework that would load all Animals for a specific region. The easiest thing to do is to create 3 Entities Region, AnimalCategory, and Animal and use LINQ to load the data.
But let's say I am not interested in loading any AnimalCategory information and define an Entity class just to represent AnimalCategory so that I can do the JOIN. How can I do this with Entity Framework? Even with many of its Mapping functions I still don't think this is possible.
In non Entity Framework solutions this is easy to accomplish by using INNER JOINs in SPs or inline SQL. So what are my options in Entity Framework? Shall I pollute my data model with these useless tables just so I can do a JOIN?
It's a matter of choice I guess. EF choose to support many-to-many associations with transparent junction tables, i.e. where junction tables only have two foreign keys to the associated entities. They simply didn't choose to support this far less common "skipping one-to-many-to-many" scenario in a similar manner.
And I can imagine why.
To start with, in a many-to-many association, the junction table is nothing but that: a junction, an association. However, in a chain of one-to-many (or many-to-one) associations it would be exceptional for any of the involved tables to be just an association. In your example...
Animal → AnimalCategory → Region
...AnimalCategory would only have a primary key (Id) and a foreign key (RegionId). That would be useless though: Animal might just as well have a RegionId itself. There's no reason to support a data model that doesn't make sense.
What you're after though, is a model in which the table in the middle does carry information (AnimalCategory.Name), but where you'd like to map it as a transparent junction table, because a particular class model doesn't need this information.
Your focus seems to be on reading data. But EF has to support all CRUD actions. The problem here would be: how to deal with inserts? Suppose Name is a required field. There would be no way to supply its value.
Another problem would be that a statement like...
region.Animals.Add(animal);
...could mean two things:
add an Animal and a new AnimalCategory, the latter referring to the Region.
Add an Animal referring to an existing AnimalCategory - without being able to choose which one.
EF wouldn't want to choose for some default behavior. You'd have to make the choice yourself, so you can't do without access to AnimalCategory.
Related
I am developing for an existing application which uses a SQL database that is used by two applications. One uses Entity Framework to connect to the database. The other uses LINQ-to-SQL. The SQL database is designed so that there are some tables showing many-to-many relationships between rows in two tables. Entity Framework seems not to import these tables, apparently because it has some object-oriented idea for how many-to-many relationships ought to be represented. So far, the Entity Framework application has not needed to know about those tables, but now it should. I don't know how that works, and I am concerned that even if I learn about Entity Framework's exciting new way to represent these relationships, that it won't cooperate nicely with the other application or the database which is designed to use the many-to-many table.
I.e., there is a table of Foos, and a table of Bars, and then a table with Foo and Bar Ids that lists which Foos relate to which Bars, and I don't want to stop using this relationship table, particularly because there is another LINQ application that heavily uses this relationship table.
Questions:
If I learn to use Entity Framework's many-to-many system, will it use and update the many-to-many table that the other application uses?
If not, what is a good way to get Entity Framework to not ignore the many-to-many relationship table, so I can write code to use the existing table?
Yes, Entity Framework will manage your many-to-many tables for you. Pure link tables (that only have two foreign key columns) in EF are represented as relationships as opposed to POCO objects. The way this is done is that you tell EF that there is a relationship between two of your objects and that table X is where this relationship is stored. As an example in EF 4.1. which is what I'm currently using this is done like so:
modelBuilder.Entity<Foo>() //Let me tell you about Foo...
.HasMany(f => f.Bars) //The property in the Foo class that links to Bar objects is Bars
.WithMany(b => b.Foos) //The property in the Bar class that links to Foo objects is Foos
.Map(m => {
m.MapLeftKey("FooID"); //Name of the foreign key column in the link table for Foo
m.MapRightKey("BarID"); //Name of the foreign key column in the link table for Bar
m.ToTable("FooBar"); //Name of the link table
});
You can then make changes to this table by linking/unlinking objects in your code. You pretty much do something like
myFoo.Bars.Add(myBar); //Add a row to the link table
myFoo.Bars.Remove(myBar) //Delete a row from the link table
For a full implementation you should google your version of EF.
In case of link tables that contain extra columns (for example a creation date) they are represented by a POCO just like all the other tables. If you're really paranoid about EF's ability to manage your link tables you can force it to go this route by adding a unique id column to your pure link tables, but I'd definitely advice against it.
Think of it this way: EF has been around for a while now and has achieved a certain degree of maturity. Combine this with the fact that many-to-many relationships are not exactly rare in databases. Do you really think the designers of EF haven't dealt with your case?
While creating a schema from a database many-to-many relationships between tables are not created.
Is this a principal problem?
Is it possible to detect from the table structure that many-to-many relationships exist and create the respective code in schema classes automagically?
It is indeed a somewhat fundamental problem -- many_to_many is a "relationship bridge" and not a "relation." The documentation explains that "the difference between a bridge and a relationship is, that the bridge cannot be used to join tables in a search, instead its component relationships must be used."
On the other hand, this means that if the real relationships are correctly discovered it should be straightforward to add the many-to-many relationships automatically: First, search for tables that have two or more has_many relationships. Then, for each pair of such relationships, create a many-to-many relationship bridge. (Of course, one might hope that DBIx::Class would do this itself.)
The problem with developing this kind of code is that many tables that contain multiple references are not many-to-many tables, and have multiple references for other reasons. For instance, I'll make up a schema for some fictional app where something could be regarded as a many-to-many table, when it is not.
create table category (
id primary key,
...
);
create table sub_category (
id primary key,
category references category(id),
...
);
/* EDIT:
This is the table that could be regarded as many_to_many
by an automated system */
create table product (
id primary key,
category references category(id),
sub_category references sub_category(id),
...
);
Something could be built this way for ease of use, without having to do multiple table joins in the database on a website, especially when considering speed. It would be difficult for a piece of code to say definitively 'this is not a many_to_many' situation, while the developer should be able to easily figure it out, and add in the many_to_many line below the checksum.
I consider DBIX::Class schema outputs a good starting point, and little more, especially when working with auto numbering in non-MySQL databases, among other things. I often need to modify above the "Don't modify above this line" stuff (although many_to_many can obviously go below that checksum, of course.
In trying to normalize a database schema and mapping it in Entity Framework, I've found that there might end up being a bunch of lookup tables. They would end up only containing key and value pairs. I'd like to consolidate them into one table that basically has two columns "Key" and "Value". For example, I'd like to be able to get Addresses.AddressType and Person.Gender to both point to the same table, but ensure that the navigation properties only return the rows applicable to the appropriate entity.
EDIT: Oops. I just realized that I left this paragraph out:
It seems like a TPH type of problem, but all of the reading I've done indicates that you start with fields in the parent entity and migrate fields over to the inherited children. I don't have any fields to move here because there would generally only be two.
There are a lot of domain-specific key-value pairs need to be represented. Some of them will change from time to time, others will not. Rather than pick and choose I want to just make everything editable. Due to the number of these kinds of properties that are going to be used, I'd rather not have to maintain a list enums that require a recompile, or end up with lots of lookup tables. So, I thought that this might be a solution.
Is there a way to represent this kind of structure in EF4? Or, am I barking up the wrong tree?
EDIT: I guess another option would be to build the table structure I want at the database level and then write views on top of that and surface those as EF entities. It just means any maintenance needs to be done at multiple levels. Does that sound more, or less desireable than a pure EF solution?
Table per hiearchy demands that you have one parent entity which is used as base class for child entities. All entities are mapped to the same table and there is special discriminator column to differ type of entity stored in database record. You can generally use it even if your child entities do not define any new properties. You will also have to define primary key for your table otherwise it will be handled as readonly entity in EF. So your table can look like:
CREATE TABLE KeyValuePairs
(
Id INT NOT NULL IDENTITY(1,1),
Key VARCHAR(50) NOT NULL,
Value NVARCHAR(255) NOT NULL,
Discriminator VARCHAR(10) NOT NULL,
Timestamp Timestamp NOT NULL
)
You will define your top level KeyValuePair entity with properties Id, Key, Value and Timestamp (set as concurrency mode fixed). Discriminator column will be used for inheritance mapping.
Be aware that EF mapping is static. If you define AddressType and Gender entities you will be able to use them but you will not be able to dynamically define new type like PhoneType. This will always require modifying your EF model, recompiling and redeploying your application.
From OOP perspective it would be nicer to not model this as object hiearchy and instead use conditional mapping of multiple unrelated entities to the same table. Unfortunatelly even EF supports conditional mapping I have never been able to map two entities to the same table yet.
I'm having trouble configuring entity relationships when one entity inherits from another. I'm new to ADO Entity Framework -- perhaps someone more experienced has some tips for how this is best done. I'm using .net 4.
Database tables with fields:
Products (int ID, nvarchar Description)
FoodProducts (int ProductID, bit IsHuge)
Flavors (int ID, int FoodProductID, nvarchar Description)
There are constraints between Products and FoodProducts as well as FoodProducts and Flavors.
Using the designer I create a model from the database. The designer seems to get it right, with a 1:0..1 association between Product and FoodProduct entities, and 1:* association between Flavor and FoodProduct. No errors when I save or build.
Next I set FoodProduct entity to inherit from Product entity. Then I get errors concerning relationship between Product and FoodProduct. Ok, starting fresh, I first delete the relationship between Product and FoodProduct before setting the inheritance. But now I get errors about the relationship between FoodProduct and Flavor. So I delete and then recreate that relationship, connecting Flavor.ID to FoodProduct.ProductID. Now I get other errors.
My question is this: Should I instead be creating relationship between Flavor.FoodProductID and Product.ID? If so, I assume I then could (or should) delete the FoodProduct.ProductID property. Since my database will have many of these types of relationships, am I better off first creating the entity model and exporting the tables to SQL, or importing the database schema and then making many tweaks?
My intent is that there will be several types of products, some of which require many additional fields, some of which do not. So there may be zero or one FoodProducts records associated with each Product record. At least by my thinking, the table for each sub-type (FoodProducts) should be able to "borrow" the primary key from Products (as a FK) to uniquely identify each of its records.
You can find a screen capture here: http://img218.imageshack.us/img218/9720/entityframework.jpg (I'd embed the img but haven't earned the requisite rep' yet!)
Well, I deleted the FoodProduct.ProductID field, as it should always return the same value as Product.ID anyway. Then, as you hinted, I had to manually map the Products.ID field to FoodProducts.ProductID field. Errors resolved. I'll write a little code to test functionality. Thanks for the "observations"!
Couple of observations:
FoodProducts needs a primary key (e,g identity - FoodProductID). Are you sure it should be a 1:0..1 between Food and FoodProducts? I would have thought it should be 1:0..*. For this cardinality to work you need a unique PK on this table.
When you setup inheritance for entities, the parent entity's properties are inherited. So FoodProducts will inherit ID from the Product table.
BUT, on the physical model (database), this field still needs to be mapped to a column on the FoodProducts table - which is why you need the identity field.
After you setup inheritance, you still need to map all the columns on the derived tables. My money is on you have not mapped "ID" on FoodProducts to any column.
If you screencapped your model and show the errors you are getting it would be much easier to diagnose the issue.
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. :)