I'm trying to see if I can have a discriminator column come from another table, not part of the hierarchy. For example, imagine the following scenario:
PersonTable (PersonID, PersonTypeID, other fields)
EmployeeTable (PersonID, etc)
StudentTable (PersonID, etc)
PersonTypeTable (PersonTypeID, PersonCategoryCode)
I'm creating an abstract class Person, that is inherited into Student and Employee.
I want to have PersonCategoryCode column be the Discriminator column based on which Student or Employee objects are instantiated by EF.
Is this possible? Adding PersonTypeTable to PersonTable mapping's yields errors. PersonTypeID is unfortunately too dynamic to put into static edmx mapping.
Related
I am using EF 6 Code-First, table per type, and I have two concrete classes Group and User. Group has a navigation property Members which contains a collection of User. I have mapped this many-to-many relationship in EF using Fluent syntax:
modelBuilder.Entity<Group>
.HasMany<User>(g => g.Members)
.WithMany(u => u.Groups);
I would like to be able to say when a member has joined a group so that I can query for, say, the newest member(s). I am not sure of how this is best accomplished within the framework.
I see the following options:
Create and use an audit table (ie GroupMembershipAudit consisting of Group, User, join/unjoin, and DateTime
Add a column to the autogenerated many-to-many table between User and Group
Is there anything within EF to facilitate this sort of storage of many-to-many historical info like this / append columns to the many-to-many relationship?
Add a column to the autogenerated many-to-many table between User and
Group
That is not possible - auto-generated junction tables can contain only keys (that is called Pure Join Table). According to Working with Many-to-Many Data Relationships article: If the join table contains fields that are not keys, the table is not a PJT and therefore Entity Framework cannot create a direct-navigation (many-to-many) association between the tables. (Join tables with non-key fields are also known as join tables with payload.)
Create and use an audit table (ie GroupMembershipAudit consisting of
Group, User, join/unjoin, and DateTime
Actually you should create GroupMembershipAudit entity. With Code First table will be generated, you don't need to create it manually.
trying to work out how to do LINQ to Entities query for a many to many relationship which has a junction with fields table.
Below are the Domains models (I am using View models, but keeping it simple for this example).
Student Domain model
StudentID (PK)
ICollection<StudentCourse> StudentCourses
StudentCourse Domain model
StudentCourseID (PK)
StudentID (FK)
CourseID (FK)
ForAdult
ForSeniour
Description
Course Domain model
CourseID (PK)
ICollection<StudentCourse> StudentCourses
Note:
Since the junction table (i.e. StudentCourse) contains fields other than the two foreign keys, EF will create an entity for this.
Lazy Loading
I've got this working for lazy loading. The Navigation properties have been declared with the 'virtual' keyword.
The Query way - works!
var student = (from s in context.Students
where s.StudentID == id
select s).SingleOrDefault<Student>()
The Method way - works!
Student student = context.Students.Find(id);
Projection
BUT, I would prefer to do this with projection, for performance reasons, i.e. less trips to the database.
I'm really stuck on how to write up the LINQ to Entities query to return 1 student with (1 or) many StudentCourses.
I don't understand thoroughly how the Entity should be shaped, if you know what I mean.
For example, I've tried:
var myvar = from s in context.Students
from sc in s.StudentCourses
where s.StudentID == id
select s
What I require is to return an entity of Student with a collection of StudentCourses which could then be assigned to a Student and passed to the View model, then to the View.
Really would appreciate any help, as I've spent alot of time trying to solve this.
Also as a side note, I'm using the SingleOrDefault() method to cast the results of the var (IQueryable I think) to type Student. Is this the preferred way to cast?
You can get EF to eagerly load the related entities by using the Include method.
So using your LINQ example:
var student = (from s in context.Students
where s.StudentID == id
select s).Include("StudentCourses").FirstOrDefault();
And using extension methods:
var student = context.Students.Include("StudentCourses").FirstOrDefault(id);
The Student instance that is returned will have the StudentCourses collection populated with related entities. This should invoke only one SQL query that joins the tables together.
To answer your aside question: I prefer to use FirstOrDefault most of the time as above. The difference is that SingleOrDefault will expect exactly one result and throws an exception otherwise, whereas FirstOrDefault will return null if a student is not found.
Also, as the cast to Student is implicit, you don't need the <Student> type parameter.
I have this Generalization relation ship between some classes in my class diagram.
FirstClass as a base class that has Name attribute, SecondClass as a derived class that the FirstClass is it's base class and has some other attributes and finally ThirdClass that is a derived class from SecondClass and it has some other attributes too. ThirdClass as an association relation ship with Widget class, 1.*.
I used Joined strategy when I wanted to implement a Data Model from the class diagram. so the FirstTable has their Pk in the SecondTable and the SecondTable has its PK in the ThirdTable.
As you see Third table should has it's PK in Widget class. And there are some costs whenever I want to Join widget table with third table because I need to fetch Name from First Table.
Is it appropriate that I read Id from FirtTable (Actually it's base class) and put it into Widget table?
I think the problem is that if you do, you can't prevent a Widget to be related to an instance of FirstTable that is not an instance of ThirdTable.
Perhaps you could put both the ThirdTable id and the FirstTable id, but in this case you must manage security controls for consistence between both ids.
In both cases, I would advise against. If the join cost is a problem for you, you should consider using a "single table" inheritance strategy.
I have very big table in my Database and a can't modify it.
So i have BaseEntity type for table.
I have several children (entity1, entity2) and i'd like to map each type to same column ("Date") and name properties differently.
Surely i can't move all same column properties to base type cause there is about 100 columns in my super table (it's not my design i've jst need to map it)
So i have 0019 error and is there any way to solve it or EF not for me?
No. TPH requires that each property defined in derived entity is exclusive for that entity (no other entity can map to the same column). This targets more general rule in EF - each column can be mapped only once. So if you need to use some column in more entities it must be defined in parent and must have same name in all child entities.
I have a MS SQL 2008 database, and I can't change its schema. I have to work with what it is.
It has three tables that relevant to the question.
Product table. The columns are:
Id (identity, PK)
Group (NOT NULL)
SubGroup (NOT NULL)
Code (Unique, NOT NULL)
Description
Contract table. The columns are:
Id (identity, PK)
Code (NOT NULL)
Descritpion
Discount table. The columns are:
Id (identity, PK)
Type (restricted to one of the four values:
'product',
'group',
'subgroup' or
'contract') (NOT NULL)
Object (depending on the value of Type refers one of the four:
Product.Code
Product.Group
Product.SubGroup
Contract.Code) (NOT NULL)
Value (NOT NULL)
The idea is that the discount can be applied to either of the four. I'd like to reiterated, that this is the database design that I can't change.
With Entity Framework I can query the tables all right with joins but I can't use navigation properties out of the box, because navigation properties are generated based on foreign key relationships from database, and you can't define "conditional" relationship in MS SQL, where the field object relates to one table when field type contains this value and relates to another table when the value is different.
My question is this: Is this possible to define classes and mappings with Entity Framework, so that I can use navigation properties in this scenario? For example, I do Discount.Object and I receive either Contract object or Product object in response, and if this is a Product object it's retrieved on the right property.
Is this, or something similar possible, or joins is the best I can do?
You said that "this is the database design that I can't change", but without changing existing tables, can you at least add views?
If you can, you can create a view for the Discount table that has four different nullable columns for each relationship. That would map nicely in EF as four navigation properties.
After you do that, if you still want a combined column, you could add your own property to the Discount entity that will return an object by checking which of the four navigation properties is not null, and returning the linked entity.
You cannot create a relational database like this. You need separate columns for the keys to each potential parent row.