So I am working on an MVC project to put to work the studying I have been doing. I am wrestling with the concept of Database Table relationships and foreign keys. I am working on a simple ecommerce site (displays products, shopping cart, user accounts..etc).
I have the following tables to start out with:
1) Products
2) Categories
I setup the Products and Categories tables to have a ProductId and CategoryId respectively. In my MySQL db, I created a FK on the Products Table to relate to the CategoryId field on the Categories table (I am not sure this was correct to begin).
My expectations for the way the database would handle the table relationship: I didn't want the DB to do anything with the products table if I deleted a category out of the Categories table, or vise versa. The only thing would be that the category field in a Product would be blank (or default) if their category was removed.
Finally, do I have to do anything in my entity classes such as in the Products class, add the ProductId to the Category.ProductId?
Eventually, when I Orders and Users to the project, I can see a relationship where each user -> many orders -> each order has many products -> and each product is in one category.
But I am having a hard time understanding how or if I should be setting up a Foreign key relationship in the two current tables of Products and Categories and if so how to setup my entity class in relation to that FK.
Any advice.
It has been my experience (with L2S) that you don't want to specify any relationships between tables. I have simply done the PK, FK logic myself. This keeps the L2S generated objects simple and is most likely the way you worked with them before in SQL. That, at least, is the case for me.
Related
I have database with different tables. But I can't get access to some tables, and I'm curious why? For example, there is Projects table and Users table in database, and I need to get list of all users that are in specific Project by Id. These tables have many to many relationship, so there is joining table with ProjectId and UserId. It's logic to use some Join to get users that I need from table Users with help of this joining table from db. But how can I make Join when my DbContext doesn't have joining table? How can I get access to joining table using DbContext, or maybe there are some different ways to get list of users by project id?
If someone have similar problem, here is a simple way to use all the data from collections in model
var project = _db.Projects.Include(x => x.Tickets).ThenInclude(y => y.Users).SingleOrDefault(p => p.Id == id);
so after that you gonna have your model and all the data from collections that you need
ProjectUser - is joining table, use for link two tables. This tables must have (usually) Collection for access to this many-to-many data.
So you can will have collection of users for this project. And doing filtering by users from your Ticket (example from image).
Yes, exists few different ways - setting with ef fluent, or attribute, or dbcontext. Better to find complete guide for many-to-many setting in your application.
Consider two tables Foo and Bar and consider models based on them. Now consider a one-to-one relationship between them.
Foo contains has_one :bar in it's declaration so that we're able to access Bar from Foo's objects. But then what I don't understand is why Bar needs a foreign key referencing Foo?
Shouldn't it be easier if they just compare both the ids to get the result?
I'm assuming that there will be problems with comparing both ids and I want to know what the problems are.
The problem with ids is that they store auto-incremented values. Let's consider 2 tables students and projects.
Let's assume a student can have at most 1 project. Which means he can either have a project or not.
Now consider 2 students A & B.
students table
id name
1 A
2 B
now projects table
id name
1 P1
2 NULL
in this case A has a project named as P1 but B doesn't and we're creating a null entry just to maintain and match the id of records present in projects with the students but this is not feasible in the long term. If in a school there are 1000 students then we'll have may be 500 empty rows for 500 students who are not working on a project.
That's why adding a column in projects table is a feasible solution to reduce the size of the table and maintain relationships as well and also if you're going to delete a record then the new id won't be same as the previous one as id's are auto-incremented.
now projects table
id name student_id
1 P1 1
is more feasible and flexible as well. You can make it has_many as well because a student can work on multiple projects as well.
I hope this helps you.
You can't assume that the DB engine will add the same IDs to rows in different tables. You can (I would not recommend) make an app with such behavior and implement it with triggers and constraints, but this would be a very creative (in a negative sense) approach to relational databases.
I will preface it by saying I'm a database person, moving into .NET, MVC, EF etc. So I fully understand about joins and foreign keys and so on but I am struggling with the EF side.
I worked through a tutorial where we did the following:
create a Student table (StudentID and personal details), a Course table (CourseID and subject details), and an Enrollment table (EnrollmentID, student and course IDs and the grade given to that student for that class).
create Models, Controllers and Views for Student and Enrollment entities to allow editing the name of a Student, enrolling a student for a class and giving them a grade, list out all enrollments, etc.
In that tutorial the Enrollments table is (what I know as) a "bridge" table because student to class is a many-to-many relationship.
So do I only need this "intermediate" model/view/controller if it's a many-to-many scenario like this?
The actual structure I want to program is:
Article (ArticleID, title, author, summary, content...) is a member of a
Category (CategoryID, title, description, ...)
Then I want to have a drop down list (or whatever UI element) of Categories of which clicking on the selected Category will give a page with a table of Title/Author/Summary of the articles that go in that category.
If an Article can be in more than one category ("Effective Use of Catnip" could be under both "Playing Games with Cats" and "Life Hacks") is that where I would need a 'bridge' table?
Please can someone explain in simple terms - whether I am just being put off by the "many to many" nature of the data in that tutorial, or if the 'bridge' table structure is more fundamental to EF for navigating a PK/FK relationship.
I think the tutorial you are reading is trying to introduce you to the basic concepts of EF. If you have a many-to-many relationship, on database side you will always have three tables:
Student
Course
Enrollment (Student_Id, Course_Id)
On EF world, you can represent those three tables as three different entities. However, if you want to have a more "natural" way of representing the relation between Students and Courses, EF allows you to declare many-to-many as two lists:
public class Student {
... properties
virtual List<Course> Courses;
}
public class Course {
... properties
virtual List<Student> Students;
}
However, you need to instruct EF how this references on both side needs to be treated. For that, you can use fluent API. With this fluent API you can define the table name that references both tables/entities:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Student>()
.HasMany<Course>(s => s.Courses)
.WithMany(c => c.Students)
.Map(cs =>
{
cs.MapLeftKey("StudentRefId");
cs.MapRightKey("CourseRefId");
cs.ToTable("StudentCourse");
});
}
Check this article explaining the M:N relationship using fluent library API. Is really simple, and with that you don't need this additional element on the middle.
In terms of UI, you just need to select the list of courses the student want to take, or the list of students the course has to have. Is up to you how you want to present the information to the user, as both functionalities (students taking courses and courses defining its students) point to the same m:n relation.
I think a simple way can be to show a </select> list with the students, and then show the list of courses he can apply to.
The project you want to create uses the same ideas. Basically, with EF you will always need to think in terms of "tables". It is true that EF allows you to abstract your tables to entities, but still you need to follow some rules. To sum up: you are right. The "bridge" table is where you need to store the many-to-many relationships.
We have a web portal in our company that is written in asp.net MVC . right now each department have their own database but we want to avoid having multiple database because the database scheme is the same only it has diffrent data inside for each department.like each department has their own projects etc. how the database model should be changed in such a way to avoid having multiple database ?
also we want to share some elments like projects between diffrents department. how it could be done ?
The single database that you have in mind would have to have its table re-designed such that you would be able to tell, for each record in each table, what department the record relates to. Essentially you would need to add a "DeptId" column to each root-level table. By root level table I mean only the parents of foreign key relationships. Eg, if you have an OrderHeader and OrderLines, only the OrderHeader table would need to have this DeptId column. The lines relate to the header, so you won't need to add this column to the lines table as well. Alternatively, if you have a Customer table, such that each customer belongs to only one department, then you would add the DeptId column to this customer table and then you won't need it on the OrderHeader table (since order header should be referencing Customer table)
Those tables/elements that you want to share between databases just would not have the DeptId column added to them.
I have already read Entity Framework One-To-One Mapping Issues and this is not duplicate as the business rule specs are different here.
There are two tables, Invoices and Orders.
Invoices
-> InvoiceID (Primary, Auto Number)
Orders
-> OrderID (Primary, Auto Number)
-> InvoiceID (FK InvoiceID of Invoices Table)
Now the problem is, EF requires One to Many relationship for this association if names of properties are not same. If names of properties are same then it serves purpose of derived class, but here Order is not derived class or Invoice.
InvoiceID(s) are generated for every shopping cart, but OrderID(s) are only generated for paid invoices, so Every Order has InvoiceID but every Order does not have corresponding Invoice.
If I create a seperate table for this, then I have to write too much code to do it. Is there any way I can remove this restriction and let EF still process my model.
However, currently if I change the model as follow, it works
Invoices
-> InvoiceID (Primary, Auto Number)
Orders
-> OrderID (Auto Number)
-> InvoiceID (Primary, FK InvoiceID of Invoices Table)
But is this good practice? Because by definition InvoiceID of Orders table will certainly be unique, but we will be referring everywhere OrderID for comparison and lot of other references. I know I can index the property, but I dont feel this design is perfect.
What seems to be the obvious solution here is to change the 1:* association between Invoice
and Order in the EDM into a 1:1 association. However, as you experienced, the mapping will not
validate when you have a Foreign Key Association between the two entities as in your model.
The only way to map a unique foreign key association is by using an Independent Association. This is the same type of association that we had in EF3.5, where foreign keys were not supported.
To turn the foreign key association into an independent association would mean removing the InvoiceID foreign key from the Order entity and recreating the association through mappings.
To make the change to the association, you’ll need to do the following:
Delete the InvoiceID foreign key property from Order entity.
Select the Asscoation between Invoice and Order.
In the Properties window for the association, open the Referential Constraints by
clicking the ellipses next to that property.
Delete the constraint by clicking the Delete button.
Right-click the association in the Designer and select Table Mapping from the context menu.
In the Mapping Details window, click the element to expose the drop-down.
From the drop-down, select Order. The mappings should populate automatically.
Return to the Properties window for the association.
For the property called “End2 Multiplicity,” which currently has the value * Collection of Orders, change that property to 1 (One of Order) using its drop-down list.
Validate the model by right-clicking the design surface and choosing Validate. You will see that the error message related to this mapping is gone.
When encountering this problem in your application, you’ll have to decide which is more important to your model and your application logic: the foreign key scalar (e.g., Order.InvoiceID) or being able to define a 1:1 association between one entity (Invoice) and another (Order) when they are joined through a foreign key (InvoiceID).
The good news is that the new EF4.0 Lazy Loading will be still working with Independent Associations, just the Foreign key is not exposed. To get that you would have to go over to the navigation property (Invoice) and read its InvoiceID like the code below:
Order order = context.Orders.First();
int invoiceID = order.Invoice.InvoiceID;
Or you can use the code below to read it right on the Order entity withought having to Lazy Load or Eager Load the Invoice property:
int invoiceID = order.InvoiceReference.EntityKey.EntityKeyValues[0].Value;