I am struggling to wrap my head around this concept. Here is a pic of my ERD
Look at the tables 'Titles' and 'Employees'. Emp_title_id is a foreign key reference to title_id in Title. Now, there are same titles for multiple employees. So shouldn't that be a many-to-one relationship?
The argument against is that the two fields (emp_title_id in Employees and title_id in Title) have same set of values, so it is a one-to-one relationship.
Please let me know if I should think of it as set of values (one to one) or set of "cells" with different values (many to one)
Thanks
One-to-many (1:n)
An employee can have only 1 title.
A title can be given to n employees.
(You have a 1:n relationship between employee and title, as an employee can have only one title. But you have established an m:n relationship between employee and department, as an employee can work in multiple departments. In the datamodel this can be seen from the existence of the bridge table dept_emp needed to provide m:n.)
in code first, When I want to save 2 related entities (for example Country entity and City entity), first, I create an instance of Country and second create an instance of City and put object of Country to navigation of City, at last SaveChanges.in this process Database, first, create the Country and then put its Id to CountryId field of City entity and save city to database. so now, I want to do the same but with non-related entities. this means I want to send 2 entity (without relationship) to DB, that fist, first one saved and second one get its Id, and use it, at last Save...
Is there a way within the entity framework designer to duplicate an entity and then apply a filter condition to both to make them unique. Id like to retain all navigation properties and what not.
For example, say in the database I had a table of orders. I could have two entities, one called IncompleteOrders and One called Complete based on the same table, with the complete having a filter specified on the database field 'complete'.
Thanks in advance
Yes, this is called Table per Hierachy
You have one physical table, which has a special, single, scalar column which is used as a discriminator.
Like this:
OrderId OrderName IsComplete
1 Foo 1
2 Bar 1
3 FooBar 0
Where IsComplete is the discriminator (BIT column, for example), so when you setup your entities on your EDMX, you create three entities:
1. Orders
2. CompleteOrders (derives from Orders)
3. InCompleteOrders (derives from Orders)
On the table mapping for Orders, you say "Maps to CompleteOrders, when IsComplete = 1", and "Maps to InCompleteOrders, when OrderType = 0".
Good writeup on TPH/Discriminator pattern here.
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 situation where I dont want to add records to the relation table.
For example :
I have "TRIPS" entity and it has attribute for "LOCATION_ID", I am filling it when user creates a new TRIP and select a LOCATION from the LOCATIONS entity
In "LOCATIONS" entity I am allowing user to create locations and I am assigning a unique ID to each location.data will not be repeated here.
Is there any way to link the LOCATION_ID into LOCATIONS entity ,so when ever I access a trip(NSManagedObject) it automatically get LOCATIONS entity record (Object) ?
I mean automatically (Manually I can do that)
Thanks,
Raghu
If I understand correctly your question, you simply need to model differently your entities in the Core Data model, as follows. In your TRIPS entity, add LOCATIONS as a relationship, not as a property as you currently do. The relationship may either be to-one or to-many from TRIPS to LOCATIONS, depending on the constraints you want to enforce in your application, and to-one from LOCATIONS to TRIPS.
Once you do this, when you fetch objects from the TRIPS entity, they will also contain a LOCATIONS object (if you decide to use a to-one relationships) or a set of LOCATIONS objects (if you decide for the to-many relationship).