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.)
I have 2 entities.
entity 1 - People
entity 2 - books
People entity has a property which is an array of string names of their favorite books.
I need to create a relationship that somehow maps the favorite book of a person to the corresponding book entity object(s).
I am not sure how to do this.
So far I have started by creating a relationship in core data model for people by setting destination to "books" and then for the books entity creating a relationship by setting destination to "people".
I don't see or understand how this will automatically pick out each of the person's favorite books...at the end of the day they are both seperate objects. How will the people class know that for a specific people instance that this, this and this book are that person's favorite?
Person attribute as array of string names of books -- very bad idea!
You need a to-many relationship with the Book entity. That's it.
Person <------------>> Book
Then, to get an array of book titles for a particular person:
(person.books as! Set<Book>).map { $0.title }
The person can have an additional to-one relationship (e.g. called favoriteBook) to one of the books.
I'm currenty working with Core-data for an iPhone project.
But I'm a bit confused about one element.
With Core Data currently you do not need to create the intermediate table when creating many to many relationships (its all handled behind the scenes by core data)
But in my case I actually need some attributes on my many to many relationship!
For example
I have a table called Students
and another table called Lessons
a Student can be in many lessons
and a lesson can have many students
Now a standard many to many relationship will not work for me as I actually need to define more details on the join, i.e. StartDate and LeaveDate.
In a standard sql model for example my join table would be something like
StudentLessons (Studentid, LessonId, StartDate, LeaveDate )
I would need these properties as when i'm querying for information I will need the details from the join to filter my results.
How can I create this in core data and also filter for results?
I've seen folks say that you would actually create the StudentLesson entity manually in core data.
Now if I did this would I just have the attributes (Startdate, LeaveDate) and then a one to many relationship from the Student and then the Lessons table?
Student - > StudentLessons
Lesson - > StudentLessons
I guess I'm a bit confused on how I would go about making sure that the relationships and the content of the relationships are setup correctly. (i.e If I add an Student object to the StudentLessons - how would I then assign/add the Lesson.)
Sorry this is my first time playing with Core Data.
Takes a bit o getting used to when coming from a full on sql background.
You are absolutely right. The correct way to do this is to create a new entity like StudentLessons. Let's call it Attendance. It should have the startDate and endDate, and two relationships.
The relationship to the student can be many-to-many, unless it is foreseeable that startDate and endDate are always different for each student. One Attendance with its dates can have many students in it. One student can have several Attendance duties.
Student <<---->> Attendance
Clearly, the relationship to Lesson should be one-to-many. One Lesson can have different Attendance configurations, with different dates. But each Attendance belongs only to one Lesson.
Lesson <---->> Attendance
To address your question, you can make the Attendance attribute of Lesson non-optional (and vice versa), this way it will ensure that each Lesson has at least one Attendance with appropriate dates, and each Attendance has exactly one Lesson.
I think your can remove the link between Student and Lesson. Just assign an Attendance rather than a lesson. If you want a Lesson assigned to a Student without dates, just allow Attendance to have NULL as those properties.
TheTiger,
Just because Core Data will create a join table for you, that doesn't mean you have to use it. Maintaining which student succeeds with which lesson is just the same except you will create the intermediate entity and then use the appropriate setters to build the relationships.
You will have to use more key paths and do relationship prefetching but those are straightforward to do.
Andrew
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).