I have two tables with names User and Role that have many to many relationship with each other. I have made this relationship with entity in MVC.
Now I want UserRole navigation table that has many to many relation with itself.
Thus UserRole has many to many relation with UserRole.
How can I implement this model?
Related
Are there ways to make one-to-many (abstract) relation without using SQL foreign keys?
I know it could be made by joining 2 non-related tables. But is it possible to use EF Core default navigation tools?
I think you want that because you can't add a foreign-key to your entity
So, you can solve that so:
You have a entity
Group
and you want a
List<User>
but your User can't be assigned to one group.
So you create a
GroupUser
entity which has a foreign key for the
List<GroupUser>
and a navigation property to the User
This is called a many-to-many relationship
Is there a way to map multiple foreign keys to the same navigation property of an entity? I have a table with a self-referencing many to many relationship defined by a relationship table.
For example, I have a Person table and I want to define the different related people. I created another table, PersonRelationship, that defines those relationships with 2 foreign keys to the Person table. It doesn't matter if the person is referenced in the first or the second foreign key just that the person is in one. I would like my Person entity to have a list of all the relationships but instead it has two lists of relationships: one in which the person is in the first foreign key and another where it is the second foreign key.
How can I map these two foreign keys so that they map to the same list in the Person entity? (I am using a database first approach)
I'm trying to workout the best database design for the following :
1) I have two (or more) tables in the database. "Sports", "Teams", "Leagues", etc..
2) Each of these tables can have a one to many relationship with another table in this case "Feeds"
The idea being that various database entities can each have a list of associated "Feeds"
It seems wrong to me to have multiple foreign key columns on the "Feeds" table, one for each of the tables (Sport, League, etc..), so my question is how best to model this?
It's worth mentioning that each feed can only belong to one of the other tables. A feed can't be associated with a "Sport" and a "League"
I've considered the following :
1) Add an additional column to each of the "Sport" "League" etc.. tables with a GUID.
2) Add another column to the "Feeds" table also with a GUID and populate this with the GUID from my other table, then query on this.
This would allow me to have multiple tables referencing the same column of the "Feeds" table but is this any better than having multiple nullable foreign keys, one for each table?
It is not a bad idea to have a foreign key pointing to Feeds in several tables. Foreign keys can be used to handle the 1:n relation (one-to-many).
It seems wrong to me to have multiple foreign key columns on the
"Feeds" table, one for each of the tables (Sport, League, etc..), so
my question is how best to model this?
Why do you think it is not a good practice, what would be the downside of this design?
I'm new to Entity Framework and just experimenting...
Consider 3 db tables where Person is a base table. I want the Employe table to enherit from Person, storing employee specific info. It seems that EF requires that PersonId also be the PK of the Employee table, so I made EmployeeID a unique index.
Next I added a table, Application, which stores one record for every software application that the Employee supports, creating a foreign key from Application.EmployeeId to Employee.EmployeeId.
However, EF doesn't seem to recognize relationships that involve unique indexes, but only Primary Keys.
What I can do is create a relationship from Application.PersonId to Person.PersonId, however, only Employees can be responsible for an Application, so it seems more natural to me to have Application as a "child" of the Employee table rather than the Person table.
Is this possible in EF?
You can build your relation between Employee (PersonId) and Application (EmployeeId). In such case the integrity should work as you expect because only PersonIds in Employee table will be only for existing employees. EF has currently no support for unique keys.
I have a entity and a many to many table associate with it. Because the many2many table have a nchar field, it can not be mapped to a association in EFv1. Is that possible to create a Entity hold both the original entity and the nchar field? Thanks!
You can map such a table, but not as a many to many association with properties, because associations in the EF don't have properties. Instead, you'll have two entities with one to many associations to a third entity representing the association.