I have following structure in Oracle database:
Course(CourseId, Name)
->Student(StudentId, Name, Comment, CourseId)
->Subject(SubjectId, Name, SubjectComment, CourseId)
Student contains some of Primitive Properties (StudentId, Name, CourseId, Comment) and Navigation Property (Courses [Linked with DTO name Course on CourseId]).
Table structure is also same as Entity structure and currenly using Explicit loading to extract the data from Oracle database, using LoadProperty and Load.
I need to load the Collection and Object with selected property, as Load Student with StudentId and Name (without Comment column).
LoadProperty(Student, s => s.Courses), load only CourseId (don't load Name primitive property in Course DTO). So, Student.Courses.First().CourseId will be a value and Name will be null as intentionally excluded from loading from database.
LoadProperty(Course, c => c.Subjects) load only SubjectId without Name property, even don't go to database to load.
Is there any way to Include/Exclude the Primitive types to load?
Not with load property and not with entities. You must create custom query and use projection to load only selected columns:
var student = from s in context.Students
select new {
StudentId = s.StudentId,
... // Other student's properties
CourseId = s.Course.Id,
SubjectIds = s.Courese.Subjects.Select(s => s.Ids)
};
Related
Can I use Entity Framework to save changes to a view?
I have an entity which is mapped to a View.
[Table("MyView")]
public class MyEntity
{
public long MyEntityId { get; set; }
public string Name { get; set; }
}
The View itself is like this:
CREATE VIEW MyView AS
SELECT
t.MyEntityId,
t.Name,
FROM
MyTable t
Would I be able to use Entity Framework change tracking to save changes to this View? So is something like this possible:
var record = Context.MyEntity.Where(e => e.MyEntityId == 150).FirstOrDefault();
record.Name = "New Name";
Context.SaveChanges();
Looks like Entity Framework does not care if the Entity is mapped to a View or Table... it would just create the same update script. For the example above EF generates the following script:
UPDATE [MyView] SET [Name]=#gp1 WHERE [MyEntityId] = 150
-- #gp1: 'New Name' (Type = String, IsNullable = false, Size = 8)
So EF does not introduce any additional limitation for updating a View... but we still have the RDBMS specific limitations for updating a View... as an example, in SQL Server a view can be updated subject to the following limitations:
If the view contains joins between multiple tables, you can only insert and update one table in the view, and you can't delete rows.
You can't directly modify data in views based on union queries. You can't modify data in views that use GROUP BY or DISTINCT statements.
All columns being modified are subject to the same restrictions as if the statements were being executed directly against the base
table.
Text and image columns can't be modified through views.
There is no checking of view criteria. For example, if the view selects all customers who live in Paris, and data is modified to
either add or edit a row that does not have City = 'Paris', the data
will be modified in the base table but not shown in the view, unless
WITH CHECK OPTION is used when defining the view.
I'm new in Grails. I have a problem with generation association many to one and one to many between two tables. I'm using postgresql database.
Employee.groovy
class Employee {
String firstName
String lastName
int hoursLimit
Contact contact
Account account
Unit unit
char isBoss
static hasMany = [positionTypes:PositionType, employeePlans: EmployeePlan]
}
EmployeePlan.groovy
class EmployeePlan {
AcademicYear academicYear
HourType hourType
int hours
float weightOfSubject
Employee employee
static belongsTo = [SubjectPlan]
}
I'd like to have access from employee to list of employeePlans and access from EmployeePlan to Employee instance. Unfortunately GORM generates only two tables Employee and EmployeePlan with employee_id. I don't have third table which should have two columns employee_id and employee_plan_id. Could you help me ?
I think your setup is correct, as you write from Employee class you can access to a collection of EmployeePlan (take care, that if you don't explicitly define EmployeePlan like a List, it will be a Set by default) and from EmployeePlan you can access Employee.
If you need List, you can define it like that:
class Employee {
String firstName
String lastName
int hoursLimit
Contact contact
Account account
Unit unit
char isBoss
//explicitly define List
List<EmployeePlan> employeePlans
static hasMany = [positionTypes:PositionType, employeePlans: EmployeePlan]
}
But back to your question. You'd like to have join table between Employee and employeePlan, but why? Its not necessary, since you have bidirectional mapping with sets (unordered), grails will not create a join table. Can you explain why do you need it? In grails the references will be auto-populated, so I don't see any issue here.
If need to preserve order of employeePlans, then define it as List, shown above, and grails will create a join table with corresponding indexes.
have you read the ref-doc? it gives you the answer immediately:
class Person {
String firstName
static hasMany = [addresses: Address]
static mapping = {
table 'people'
firstName column: 'First_Name'
addresses joinTable: [name: 'Person_Addresses',
key: 'Person_Id',
column: 'Address_Id']
}
}
Say I have two entities with about 20 properties per entity and a Many-to-Many relationship like so:
User (Id int,Name string, .......)
Issue (Id int,Name string, .......)
IssueAssignment (UserId,RoleId)
I want to create a new Issue and assign it to a number of existing Users. If I have code like so:
foreach(var userId in existingUserIds)
{
int id = userId
var user = _db.Users.First(r => r.Id == id);
issue.AssignedUsers.add(user);
}
_db.Users.AddObject(user);
_db.SaveChanges();
I noticed it seems terrribly inefficient when I run it against my SQL Database. If I look at
the SQL Profiler it's doing the following:
SELECT TOP(1) * FROM User WHERE UserId = userId
SELECT * FROM IssueAssignment ON User.Id = userId
INSERT INTO User ....
INSERT INTO IssueAssignment
My questions are:
(a) why do (1) and (2) have to happen at all?
(b) Both (1) and (2) bring back all fields do I need to do a object projection to limit the
fields, seems like unnecessary work too.
Thanks for the help
I have some possible clues for you:
This is how EF behaves. _db.Users is actaully a query and calling First on the query means executing the query in database.
I guess you are using EFv4 with T4 template and lazy loading is turned on. T4 templates create 'clever' objects which are able to fixup their navigation properties so once you add a User to an Issue it internally triggers fixup and tries to add the Issue to the User as well. This in turns triggers lazy loading of all issues related to the user.
So the trick is using dummy objects instead of real user. You know the id and you only want to create realtion between new issue and existing user. Try this (works with EFv4+ and POCOs):
foreach(var userId in existingUserIds)
{
var user = new User { Id = userId };
var _db.Users.Attach(user); // User with this Id mustn't be already loaded
issue.AssignedUsers.Add(user);
}
context.Issues.AddObject(issue);
context.SaveChanges();
I implemented inheritance with a discriminator field so all my records are in the same table. My basetype is Person (also the name of the table) and Driver and Passenger inherit from it. I receive instances of the correct type (Driver and Passenger) when I perform a query on the object context to Person. example:
var q = from d in ctx.Person
select d;
But I also create a function that calls a stored procedure and mapped the output of the function to the type Person. But now I get a list of Person and not Drivers or Passengers when I execute this method.
Anybody an idea how to solve this or is this a bug in EF4?
AFAIK, you can't use discriminator mapping (e.g TPH) when dealing with stored procedure mappings.
The stored procedure must be mapped to a complex type or custom entity (e.g POCO), the mapping cannot be conditional.
What you could do is map it to a regular POCO, but then project that result set into the relevant derived type (manual discrimination).
E.g:
public ICollection<Person> GetPeople()
{
var results = ExecuteFunction<Person>(); // result is ObjectResult<Person>
ICollection<Person> people = new List<Person>();
foreach (var result in results)
{
if (result.FieldWhichIsYourDiscriminator == discriminatorForDriver)
{
people.Add((Driver)result);
}
// other discriminators
}
}
If your always expecting a collection of one type (e.g only Drivers), then you wouldn't need the foreach loop, you could just add the range. The above is in case you are expecting a mixed bag of different people types.
Would be interested to see other answers, and if there is a better way though - but the above should work.
I am trying to achieve the following using Entity framework 4.0 and self-tracking entities:
1) The client application request a book form the server by providing an ISBN number
2) The server performs a query on its database to see if the book is already present
3a) If the book is in the database, it returns it.
3b) If the book is not in the database, it will query Amazon for info, extract the required attributes, create a new book, store it in the database, and return it to the client
Now, 3b) is where the problems are... I can't find any information on how I can create an entity object (a book) on the server side, add it to the context and store it in the database. I have tried all sorts of things:
public class BookBrowserService : IBookBrowserService {
public Book GetBook(string ISBN) {
using (var ctx = new BookBrowserModelContainer()) {
Book book = ctx.Books.Where(b => b.ISBN == ISBN).SingleOrDefault();
if (book == null) {
book = new Book();
book.ISBN = ISBN; // This is the key
book.Title = "This title would be retrieved from Amazon";
Author author = new Author();
author.Name = "The author's name would be retrieved from Amazon";
book.Authors.Add(author);
ctx.Books.AddObject(book);
ctx.SaveChanges(); // This one always throws an exception...
}
return book;
}
}
}
Could anyone tell me what I am doing wrong?
It looks like the problem is related to the EDMX model.
I have a Book entity and an Author entity, with a many-to-many relationship.
The Book entity's Key is ISBN, which is a string of Max length 13.
StoreGeneratedPattern is set to None.
The Author entity's Key is Id, which is a Guid.
StoreGeneratedPattern is Identity.
The exception message is:
"Cannot insert the value NULL into column 'Id', table 'BookBrowser.dbo.Authors'; column does not allow nulls. INSERT fails. The statement has been terminated. "
But since StoreGeneratedPattern is set to Identity, shouldn't an Id value be created automatically?
Thanks,
Peter
It looks that the problem was that I used a Guid as Key in combination with StoreGeneratedPattern = Identity.
When I set StoreGeneratedPattern to None and create my own Guid using Id = Guid.NewGuid(), the problem is gone.
Apparently, the SQL server cannot generate Guids...
you can use StoreGeneratedPattern=Identity, but generated sql script based on your edmx doesn`t contain newid() in describing primary key(GUID). you can do this manually in generated sql script. 'BookId uniqueidentifier NOT NULL
DEFAULT newid()'. So id value will create GUID automatically.