I use Database first approach. I have the following tables in my database:
[Country](ID, Name)
[Status](ID, Name)
[User](ID, Name)
[Object](ID, StatusID, Name)
[ObjectXCountries](ObjectID, CountryID)
[ObjectXUser](ObjectID, UserID)
Now Entity Framework creates these entities:
Country { ID, Name }
User { ID, Name }
Status { ID, Name }
Object { ID, StatusID, Status, Name, Countries, Users }
To edit a list of objects UI needs the following JSON structure:
{
cnt: [ { id: 1, name: 'some country', ...}, ],
usr: [ { id: 1, name: 'some user', ...}, ],
st: [ { id: 1, name: 'some status', ...}, ],
objects: [ { id: 1, name: 'some name', st: 1, cnt: [ 1, 2, 3 ], usr: [ 1, 2, 3] }, ... ]
}
Getting a list of countries, users and statuses is trivial, but how do I get a list of object with ids connected to them? Right now I have the following code, which results in a very inefficient query:
ctx.Object.Select(rec => new ObjectDTO {
ID = rec.ID,
Name = rec.Name,
StatusID = rec.StatusID,
UserIDs = rec.Users.Select(usr => usr.ID).ToArray(),
CountryIDs = rec.Countries.Select(cnt => cnt.ID).ToArray()
});
It translates to sql, which is dependent on the number of objects, it selects all objects and then it creates a select query for each of the objects to get a list of user ids and countries. If I have a hundred objects, it will result in 101 queries. First one to get a list of objects and 100 to get connected ids. (UPD: 201 queries, for each entity there are two additional queries to get user ids and countries ids).
Is it possible to select this info using only 3 select queries? It's easy to do in sql, but I want to know how I can do it in Entity Framework.
UPD: In SQL I would just write 3 select statements:
SELECT * FROM [Object]
SELECT * FROM [ObjectXCountries]
SELECT * FROM [ObjectXUser]
Call this query from code and then join three result sets on ObjectID. Maybe there's some way to make tables [ObjectXCountries] and [ObjectXUser] available in entity context, so I can write a similar code?
UPD: I use Entity Framework 6.2.
Ok, after googling a lot and trying out different ways, I have found a simple and working solution.
Multiple queries are the result of Lazy Loading. If I "Include" my navigational properties it will get everything in one query. This can be achieved with the following code:
ctx
.Object
.Include("Users")
.Include("Countries")
.Select(rec => new ObjectDTO {
ID = rec.ID,
Name = rec.Name,
StatusID = rec.StatusID,
UserIDs = rec.Users.Select(usr => usr.ID).ToArray(),
CountryIDs = rec.Countries.Select(cnt => cnt.ID).ToArray()
});
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