I have two models:
Location
Id
FacilityMnemonic
Name
Facility
Id
FacilityMnemonic
Name
Location's FacilityMnemonic can be null. It's possible that the location may have a facility mnemonic that does not have a facility. I need a way to structure these entities so that I can do some form of
_context.Locations.Where(CONDITIONS HERE).Include("Facility").ToList();
And then be able to package that up in a nice json response for my web API. I cannot use a foreign key, however, because the constraint will fail if there is a mnemonic with no matching facility. Having trouble determining the right way to do this with entity framework. Any guidance appreciated.
You cannot use include without relationship in SQL server.
You can use this instead
var list = (from a in _context.Location.Where(CONDITIONS HERE)
from b in _context.Facility.Where(CONDITIONS HERE related to Location).DefaultOrEmpty()
select new Location() {
Id = a.Id,
Name = a.Name,
Facilities = b
}).ToList();
Related
Given the following code, how can I add an element to one of the properties of an entity without knowing its Id and retrieving it from the database?
public async Task BookInPersonVisitAsync(Guid propertyId, DateTime dateTime, CancellationToken token)
{
var entity = new OnBoardingProcessEntity{ ExternalId = propertyId };
DbContext.OnBoardingProcesses.Attach(entity);
entity.OnBoardingProcessVisits.Add(new OnBoardingProcessVisitEntity
{
DateTime = dateTime,
Occurred = false
});
await DbContext.SaveChangesAsync(token);
}
ExternalId is just a guid we use for external reference. This doesnt work cause it does not have the id set, but without hitting the database we cant have it.
With entity framework if you have to reference an entity (referencedEntity) from another entity (entity) you have to know referencedEntity.
Otherwise you can add just add the entity setting the referencedEntity to null.
To know the referencedEntity or you know the Id or you have to retrieve it in some ways (from the database).
In SQL (DML) if (and only if) ExternalId is a candidate key noy nullable you can insert the OnBoardingProcessVisit record with a single roundtrip but the insert statement will contain an inner query.
OnBoardingProcessVisit.OnBoardingProcess_Id = (
SELECT
Id
FROM
OnBoardingProcess
WHERE
ExternalId = #propertyId)
EDIT
No way to generate that query with EF. You can have a look to external components (free and not free, for example EntityFramework Extended but in this case I think that doesn't help).
In this case I probably would try to use standard entity framework features (so 1 roundtrip to retrieve the OnBoardingProcess from the ExternalId).
Then, if the roundtrip is too slow, run the SQL query directly on the database.
About performances (and database consistency) add a unique index on OnBoardingProcess.ExternalId (in every case).
Another suggestion if you decide for the roundtrip.
In your code, the entity will be a proxy. If you don't disable lazy load, using your code you will do one more roundtrip when you will access to property
entity.OnBoardingProcessVisits (in the statement entity.OnBoardingProcessVisits.Add).
So, in this case, disable lazy load or do the same using a different way.
The different way in your case is something like
var onBoardingProcessVisitEntity new OnBoardingProcessVisitEntity
{
DateTime = dateTime,
Occurred = false,
OnBoardingProcess = entity
});
DbContext.OnBoardingProcessVisits.Add(onBoardingProcessVisitEntity);
await DbContext.SaveChangesAsync(token);
I am new to EF so please bear with me. I am using Entity Framework with an existing database in which the relationships are not defined.
Using EF, I am able to produce the model, but obviously the 'navigational properties' are not working. Is there a way I can specify the mapping between the entities?
For example, in my Product entity, I have a field CategoryID_fk (maps to Category entity). But since the relationships are not defined, I cannot load a Category while loading a Product entity.
Can someone guide me in this regard?
I do understand that it would be preferable to refactor our database but I am unable to do that now. Thanks in advance.
This link is very useful.http://www.entityframeworktutorial.net/
I think field names are not suitable conventions.Try to change field name as CategoryId or Category_CategoryId .I think it will work
Yes, you can do a group join. Basically you will retrieve Products and Category in separate queries and take an approach as indicated in this answer.
Something along these lines:
var productCategoryList = products.GroupJoin
(
categories,
p=> p.productId,
c=> c.CategoryId,
(p, c) => new ProductCategory
{
//Create your Product Category model here
}
).AsEnumerable();
I don't know whether I should be drawing parallels, but unfortunately, that's the only way I can express my issue.
In SQL, suppose we have two tables:
Employee with columns Employee ID, Employee Name, Dept. ID
Deptartment with columns Dept. ID, Dept Name
The Dept ID. in the Employee table is a foreign key with that in the Department table.
Now suppose I want to fetch the following columns:
Employee ID, Employee Name, Department Name
using a SQL such as:
SELECT A.EMPLOYEE_ID, A.EMPLOYEE_NAME, B.DEPT_NAME
FROM EMPLOYEE A, DEPARTMENT B
WHERE A.DEPT_ID = B.DEPT_ID
How would one do this using Core Data in Swift? I guess I'm getting confused by only seeing references to
NSFetchRequest(entityName: entityName)
where the entityName refers to a single entity (table in relational DB lingo)
I'd really appreciate any pointers or examples that can help me clear my doubts.
Thanks
It is certainly possible to create a fetch request that is equivalent to your SQL query. More complex queries can be difficult if not impossible to achieve with a single fetch request. But I recommend trying NOT to draw parallels between CoreData and SQL, at least until you have got to grips with how it works.
To take your example, in the CoreData view of the world, Employee would be an entity with a relationship to another entity, Department. A fetch request based on the Employee entity will return an array of Employee objects, and (assuming you create subclasses of NSManagedObject for each entity) you can access the attributes with simple dot notation:
let employeeName = myEmployeeObject.employeeName
But you can use the same notation to traverse relationships equally easily:
let departmentName = myEmployeeObject.department.departmentName
You don't need to worry about joins, etc; CoreData handles that for you.
Now, suppose you try to do it "the SQL way". You can construct a fetch request based on the Employee entity, but specify "properties to fetch" which likewise traverse the relationship:
let fetch = NSFetchRequest(entity:"Employee")
fetch.propertiesToFetch = ["employeeID", "employeeName", "department.departmentName"]
For this to work, you would need to specify the "resultType" for the fetch request to be DictionaryResultType:
fetch.resultType = .DictionaryResultType
The results from this query would be an array of dictionaries containing the relevant keys and values. But the link with the underlying objects is lost. If you subsequently want to access any details from the relevant Department (or even the Employee), you would have to run a new fetch to get the object itself.
I'm having problems getting the mapped table name when using fluent api. When you use data annotations it's easy because you can use the TableAttribute in order to get the table name from an specific entity, but when I use fluent api I can't find a way to do that.
I need it because I'm implementing an audit trail overriding the SaveChanges on the dbContext.
Thanks
I figure it out.
In order to get information about the Entity when using Fluent API I did this:
// get the actual entity Set
// DataSpace.SSpace = Storage part of the model has info about the shape of our tables
var entitySet = workspace.GetItems<System.Data.Entity.Core.Metadata.Edm.EntityContainer>(System.Data.Entity.Core.Metadata.Edm.DataSpace.SSpace)
.SelectMany(e => e.EntitySets).ToList().Where(a => a.Name == entityTypeName).FirstOrDefault();
And finally:
// Get table name
string tableName = entitySet.Table;
Hope this can help others!
I was wondering with Entity Framework 4.1 code first how do you guys handle queries that involve an existing aspnet_Users table?
Basically I have a requirement for a query that involves the aspnet_Users so that I can return the username:
SELECT t.Prop1, u.Username
FROM Table1 t
INNER JOIN aspnet_User u ON t.UserId = u.UserId
Where t.Prop2 = true
Ideally in linq I would like:
from t in context.Table1
join u in context.aspnet_Users on t.UserId equals u.UserId
where t.Prop2 = true
But I'm not sure how to get aspnet_Users mapping to a class User? how do I make aspnet_Users part of my dbset ?
Any help would be appreciated, thanks in advance
Don't map aspnet_Users table or any other table related to aspnet. These tables have their own data access and their own logic for accessing. Mapping these tables will introduce code duplication, possible problems and breaks separation of concerns. If you need users for queries, create view with only needed information like id, user name, email and map the view. The point is that view will be read only, it will contain only allowed data and your application will not accidentally modify these data without using ASP.NET API.
First read Ladislav's answer. If you still want to go ahead : to do what you want would involve mapping the users and roles and members tables into the codefirst domain - which means writing a membership provider in code-first.
Luckily there is a project for that http://codefirstmembership.codeplex.com/ although its not a perfect implementation. The original is VB, look in the Discussion tab for my work on getting it running in c# MVC.
I'm working with the author on a better implementation that protects the membership data (password, last logged on date, all of the non-allowed data) but allow you to map and extend the user table. But its not ready yet!
You don't really need to use Entity Framework to access aspnet_membership provider accounts. You really just need to create an instance of the membership object, pass in a unique user identifier and a Boolean value indicating whether to update the LastActivityDate value for the user and the method returns a MembershipUser object populated with current values from the data source for the specified user.
You can then access the username by using the property of "Username".
Example:
private MembershipUser user =
Membership.GetUser(7578ec40-9e91-4458-b3d6-0a69dee82c6e, True);
Response.Write(user.UserName);
In case you have additional questions about MembershipProvider, you can read up on it on the MSDN website under the title of "Managing Users by Using Membership".
Hope this helps you some with your requirement.