I'm a long time dev, but still kind of new to LINQ. I'm OK when dealing with one set of object, but things get tougher when I need to pull from several sources, and I could use some guidance in getting what I need here.
I have three tables in my database, two related tables and one that holds the PK/FK to tie them together. So something like:
Users
UserID
UserName
Surveys
SurveyID
SurveyName
UserSurveys
UserID
SurveyID
I am using EF and so all of this data has been pulled into Objects.
So... what I want to do is return a List of all Surveys that are associated with a given User. So something like (pseudo-code):
// currentUserID = the UserID I need to get matching Surveys for
var surveys = from Survey where (s => s.SurveyID == UserSurvey.SurveyID && UserSurvey.UserID == currentUserID);
I assume I need to make a sub-query and use a Contains() or something like that, but I keep tripping over myself. Help?
Should be something like this:
from us in UserSurveys
where us.UserId == currentUserID
join s in Surveys on us.SurveyID equals s.SurveyID
select s
If this is EF you should be able to do someUser.Surveys.
Assuming your database and entity model has all of your FK references you should be able to do something like this....
// currentUserID = the UserID I need to get matching Surveys for
var surveys = from s in Survey
where s.User.UserID == currentUserID
select s;
Related
I wonder if it is possible to select by a condition in a preloaded object. For example, I have tables User and Profile (one to one). So I need to get all Users with Profiles where sex is female.
I thought it can be done by something like this:
Preload("UserProfile").Where("user_profile.sex = ?", "female")
But it returns something like:
pq: missing FROM-clause entry for table \"user_profile\"
Preloading doesn't join the tables specified. You need to explicitly join the table in question:
Preload("UserProfile").Joins("LEFT JOIN user_profile ON user.id = user_profile.user_id")
.Where("user_profile.sex = ?", "female")
...Assuming your primary key is called id, and foreign key is called user_id.
I also faced the same issue in a recent project. I found the below solution to work for me.
var users []User
Preload("UserProfile","sex = ?","female").Find(&users)
and then check for
user.profile!=nil
The issue in this approach is it will load all users.
But in your case, it can be another way around.
var profiles []Profile
Preload("User").where("sex = ?","female").Find(&profiles)
I hope this will solve your problem.
I have this method I wrote as:
private void GetReceivedInvoiceTasks(User user, List<Task> tasks)
{
var taskList = from i in _db.Invoices
join a in user.Areas on i.AreaId equals a.AreaId
where i.Status == InvoiceStatuses.Received
select i;
}
Basically I was trying to get all the Invoices in the database in the user's area that have a status of received. I don't understand LINQ too well right now.
I'm getting the error:
base {System.SystemException} = {"Unable to create a constant value of
type 'Models.Area'. Only primitive types ('such as Int32, String, and
Guid') are supported in this context."}
Can someone please explain to me what I'm doing wrong and how to remedy this? I can't currently understand what the problem is. If I take the join line out it's fine but I really need that line to make sure I only have invoices from the users area/areas (they can belong to more than one area). Have I got something fundamentally wrong with this query?
Entity framework does not support joins with in-memory collections. You will have to re-wire your query to make use of a Contains query with a collection of primitives instead.
Also EF currently does not support enum values so you will have to compare with an integer value (If InvoiceStatuses.Received is not an enum value ignore this part).
Both fixes combined lead to the following query approach which should result in results equivalent to your join:
int statusReceived = (int)InvoiceStatuses.Received;
var areaIds = user.Areas.Select(x=> x.AreaId).ToArray();
var taskList = from i in _db.Invoices
where i.Status == statusReceived && areaIds.Contains(i.AreaId)
select i;
I'm a total LINQ noob so I guess you'll probably have a good laugh reading this question. I'm learning LINQ to create queries in LightSwitch and what I don't seem to understand is to select an entity based on a value in a lookup table. Say I want to select all employees in a table that have a job title that is picked from a related lookup table. I want the descriptive value in the lookup table for the user to pick from a list to use as a parameter in a query, not the non-descriptive id's.
Can someone point me to an article or tutorial that quickly explains this, or give me a quick answer? I AM reading books and have a Pluralsight account but since this is probably the most extensive knowledge I will need for now a simple tutorial would help me more that watching hours of videos and read thousands of pages of books.
Thanks in advance!
Edit: this is the code. As far as I know this should but won't work (red squigly line under EmployeeTitle, error says that EmployeeContract does not contain a definition for EmployeeTitle even though there is a relationship between the two).
partial void ActiveEngineers_PreprocessQuery(ref IQueryable<Employee> query)
{
query = from Employee e in query
where e.EmployeeContract.EmployeeTitle.Description == "Engineer"
select e;
}
Edit 2: This works! But why this one and not the other?
partial void ActiveContracts_PreprocessQuery(ref IQueryable<EmployeeContract> query)
{
query = from EmployeeContract e in query
where e.EmployeeTitle.Description == "Engineer"
select e;
}
The red squiggly line you've described is likely because each Employee can have 1-to-many EmployeeContracts. Therefore, Employee.EmployeeContracts is actually an IEnumerable<EmployeeContract>, which in turn does not have a "EmployeeTitle" property.
I think what you're looking for might be:
partial void ActiveEngineers_PreprocessQuery(ref IQueryable<Employee> query)
{
query = from Employee e in query
where e.EmployeeContract.Any(x => x.EmployeeTitle.Description == "Engineer")
select e;
}
What this is saying is that at least one of the Employee's EmployeeContracts must have an EmployeeTitle.Description == "Engineer"
Try something like this:
partial void RetrieveCustomer_Execute()
{
Order order = this.DataWorkspace.NorthwindData.Orders_Single
(Orders.SelectedItem.OrderID);
Customer cust = order.Customer;
//Perform some task on the customer entity.
}
(http://msdn.microsoft.com/en-us/library/ff851990.aspx#ReadingData)
Assuming you have navigation properties in place for the foreign key over to the lookup table, it should be something like:
var allMonkies = from employee in context.Employees
where employee.EmployeeTitle.FullTitle == "Code Monkey"
select employee;
If you don't have a navigation property, you can still get the same via 'manual' join:
var allMonkies = from employee in context.Employees
join title in context.EmployeeTitles
on employee.EmployeeTitleID equals title.ID
where title.FullTitle == "Code Monkey"
select employee;
My data structure is set up this way
A user takes a number of modules
A module contains a number of courses
Here's how the relationship looks like:
How do I get a list of courses the user takes?
The query I have now is:
var courses = (from ClassEnrollment enrolment in entities.ClassEnrollment
where enrolment.UserID == UserID
join Module module in entities.Module
on enrolment.ModuleID equals module.ID
select module.Course
).ToList();
However, this doesn't result in a list of courses, but rather a list of list of courses.
How can I flatten this query to a list of distinct courses?
According to your data structure screenshot, you have a one-to-many relationship between the ClassEnrollment and Module, as well as navigational property called Module. You also have a many-to-many relationship between Module and Course, but the navigational property should be called Courses. Given your code, you want something like this:
var courses = entities.
ClassEnrollment.
Where(e => e.UserID == UserID).
SelectMany(e => e.Module.Courses).
ToList();
Your question, however, mentions a user: A user takes a number of modules, How do I get a list of courses the user takes?. I don't see any User entity anywhere else, though, so it would be nice if you could clarify. Are you using LINQ-to-SQL, btw?
Something like this:
var courses = from ClassEnrollment enrolment in entities.ClassEnrollment
from module in entities.Module
where enrolment.ModuleID equals module.ID && enrolment.UserID equals UserID
select module.Course
Use SelectMany.
You can use
courses.SelectMany(c => c);
In your query you don't need explicitly specify the type for the range variables
Or you can join course to the query
var query = from enrolment in entities.ClassEnrollment
join module in entities.Module on enrolment.ModuleID equals module.ID
join course in entities.Course on module.CourseID equals course.ID
where enrolment.UserID == UserID
select course;
var course = query.ToList();
Is this linq query efficient?
var qry = ((from member in this.ObjectContext.TreeMembers.Where(m => m.UserId == userId && m.Birthdate == null)
select member.TreeMemberId).Except(from item in this.ObjectContext.FamilyEvents select item.TreeMemberId));
var mainQry = from mainMember in this.ObjectContext.TreeMembers
where qry.Contains(mainMember.TreeMemberId)
select mainMember;
Will this be translated into multiple sql calls or just one? Can it be optimised? Basically I have 2 tables, I want to select those records from table1 where datetime is null and that record should not exist in table2.
The easiest way to find out if the query will make multiple calls is to set the .Log property of the data context. I typically set it to write to a DebugOutputWriter. A good example for this kind of class can be found here.
For a general way of thinking about it however, if you use a property of your class that does not directly map to a database field in a where clause or a join clause, it will typically make multiple calls. From what you have provided, it looks like this is not the case for your scenario, but I can't absolutely certain and suggest using the method listed above.