public IQueryable<Student> GetStudents()
{
var balance = (from StandardX in db.Standards
join StudentX in db.Students
on StandardX.StandardId equals StudentX.StandardRefId
select new { StudentX.StudentID, StudentX.StudentName, StandardX.StandardName });
return balance; // I can't handle that. how can I return? Please help.
}
Question:
I have two tables named respectively standard and student. I want to retrieve all student information via foreign key relationship. I searched everywhere regarding this topic but didn't find any suitable solution. Please help me with solving this issue.
If you have the relationship configured, try the include statement (assuming standard is the parent and 1:1, otherwise flip and/or add collection):
var balance = (from StandardX std in db.Standards.Include("Student")
select new { std.Student.StudentID, std.Student.StudentName, std.StandardName });
https://msdn.microsoft.com/en-us/library/bb896272.aspx
Related
How insert item on top table in PostgreSQL? That it is possible? In the table I have only two fields as text. First is primary key.
CREATE TABLE news_table (
title text not null primary key,
url text not null
);
I need a simple query for the program in java.
OK, this is my code:
get("/getnews", (request, response) -> {
List<News> getNews = newsService.getNews();
List<News> getAllNews = newsService.getAllNews();
try (Connection connection = DB.sql2o.open()) {
String sql = "INSERT INTO news_table(title, url) VALUES (:title, :url)";
for (News news : getNews) {
if (!getAllNews.contains(news)) {
connection.createQuery(sql, true)
.addParameter("title", news.getTitle())
.addParameter("url", news.getUrl())
.executeUpdate()
.getKey();
}
}
}
return newsService.getNews();
}, json());
The problem is that as it calls getnews method for the second time this new news adds at the end of the table, and there is no extant hronologi news. How this resolve? I use Sql2o + sparkjava.
Probably already I know. I need to reverse the List getnews before I will must contains object getnews and getallnews?
There is no start or end in a table. If you want to sort your data, just use an ORDER BY in your SELECT statements. Without ORDER BY, there is no order.
Relational theory, the mathematical foundation of relational databases, lays down certain conditions that relations (represented in real databases as tables) must obey. One of them is that they have no ordering (i.e., the rows will neither be stored nor retrieved in any particular order, since they are treated as a mathematical set). It's therefore completely under the control of the RDBMS where a new row is entered into a table.
Hence there is no way to ensure a particular ordering of the data without using an ORDER BY clause when you retrieve the data.
Im trying to select only specific attributes on the many-to-many relation users, just like in one-to-one. But using select() on belongsToMany() seem to be ignored and i'm still getting all the User attributes.
class Computer extends Eloquent {
public function users() {
return $this->belongsToMany("User")->select("email");
}
public function admin() {
return $this->hasOne("User")->select("email");
}
}
Computer::with("users")->get();
Is there a way of filtering only specified columns from related entity with belongsToMany()?
Yes, you actually can.
Computer::with("users")->get(array('column_name1','column_name2',...));
Be careful though if you have the same column name for both tables linked by your pivot table. In this case, you need to specify the table name in dot notation, tableName.columnName. For example if both users and computer has a column name id, you need to do :
Computer::with("users")->get(array('users.id','column_name2',...));
According to Taylor Otwell it is not currently possible: https://github.com/laravel/laravel/issues/2679
I have tried to use a lists('user.email') at the end of the query but I can't make it work.
Computer::with(["users" => function($query){
$query->select('column1','column2','...');
}])->get();
Im trying to call a stored procedure using Entity framework.
If I go direcly to the web api method it works fine, but when calling it from breeze it causes an exception on the metadata method.
The error is :
"Could not find the CLR type for...".
Anyone know how to fix this?
I had the very same issue, but thank God I figured out a solution. Instead of using a stored procedure, you should use a view, as Breeze recognizes views as DbSet<T>, just like tables. Say you have a SQL server table that contains two tables Customers and Orders.
Customers (**CustomerId**, FirstName, LastName)
Orders (OrderId, #CustomerId, OrderDate, OrderTotal)
Now, say you want a query that returns orders by CustomerId. Usually, you would do that in a stored procedure, but as I said, you need to use a view instead. So the query will look like this in the view.
Select o.OrderId, c.CustomerId, o.OrderDate, o.OrderTotal
from dbo.Orders o inner join dbo.Customers c on c.CustomerId = o.CustomerId
Notice there is no filtering (where ...). So:
i. Create a [general] view that includes the filtering key(s) and name it, say, OrdersByCustomers
ii. Add the OrdersByCustomers view to the entity model in your VS project
iii. Add the entity to the Breeze controller, as such:
public IQueryable<OrdersByCustomers> OrdersByCustomerId(int id)
{
return _contextProvider.Context.OrdersByCustomers
.Where(r => r.CustomerId == id);
}
Notice the .Where(r => r.CustomerId == id) filter. We could do it in the data service file, but because we want the user to see only his personal data, we need to filter from the server so it only returns his data.
iv. Now, that the entity is set in the controller, you may invoke it in the data service file, as such:
var getOrdersByCustomerId = function(orderObservable, id)
{
var query = breeze.EntityQuery.from('OrdersByCustomerId')
.WithParameters({ CustomerId: id });
return manager.executeQuery(query)
.then(function(data) {
if (orderObservable) orderObservable(data.results);
}
.fail(function(e) {
logError('Retrieve Data Failed');
}
}
v. You probably know what to do next from here.
Hope it helps.
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 have a table AccountSecurity which is a many-to-many table that relates Account entities and Securities. When I write the query below it returns all Securities that satisfy the where clause. However each Security instance in the list no longer has the reference to the AccountSecurity it came from. So when I do list[0].AccountSecurity it is empty. Is there anyway to include that information? I know I can rewrite the query to return AccountSecurities instead and use .Include("Security") on that, but I wonder if it can be done another way.
var list = (from acctSec in base.context.AccountSecurities
where acctSec.AccountId == accountId
select acctSec.Security).ToList();
UPDATE
Of course if I do two queries the graph gets populated properly, there has to be a way to do this in one shot.
var securities = (from acctSec in base.context.AccountSecurities
where acctSec.AccountId == accountId
select acctSec.Security).ToList();
//this query populates the AccountSecurities references within Security instances returned by query above
var xref = (from acctSec in base.context.AccountSecurities
where acctSec.AccountId == accountId
select acctSec).ToList();
var list = (from sec in base.context.Securities
.Include("AccountSecurity")
where sec.AccountSecurities.Any(as => as.AccountId == accountId)
select sec).ToList();
Try this:
var list = (from acctSec in base.context.AccountSecurities.Include("Security")
where acctSec.AccountId == accountId
select acctSec).ToList();
Then simply use the Security property as needed, and since it's read at the same time AccountSecurities is (single SQL with join), it will be very efficient.