Live Views How To: Create Views Based on Other Views and Create Indexes on Views
Multiple queries with parameters can often be replaced with a single “big” live view. That can help making code clear and simple and can often make it faster. This technique is based on two features of live views:
•A view can be based on other views in the same way as it can be based on base data.
•Indexes can be created for a view in the same way as indexes are created for base data.
So, instead of issuing many queries by varying a parameter value, you can create a single live view, create an index for it, and, when you need the list of items corresponding to a particular value of the parameter, you simply get those items from the index for that particular value.
It is illustrated in the LiveLinqIssueTracker demo:
The Assigned Issues form uses multiple views, a separate view for every employee: views depending on a parameter employeeID (we are giving LiveLinq to DataSet versions of the views here, Objects and XML versions are similar):
from i in _dataSet.Issues.AsLive()
join p in _dataSet.Products.AsLive()
on i.ProductID equals p.ProductID
join f in _dataSet.Features.AsLive()
on new { i.ProductID, i.FeatureID }
equals new { f.ProductID, f.FeatureID }
join e in _dataSet.Employees.AsLive()
on i.AssignedTo equals e.EmployeeID
where i.AssignedTo == employeeID
select new Issue
{
IssueID = i.IssueID,
ProductName = p.ProductName,
FeatureName = f.FeatureName,
Description = i.Description,
AssignedTo = e.FullName
};
The demo application also contains an alternative implementation of the same functionality, in the form Assigned Issues 2. That form uses a single view containing data for all employees, instead of multiple views depending on a parameter. This single view does not have parameters:
_bigView =
from i in _dataSet.Issues.AsLive()
join p in _dataSet.Products.AsLive()
on i.ProductID equals p.ProductID
join f in _dataSet.Features.AsLive()
on new { i.ProductID, i.FeatureID }
equals new { f.ProductID, f.FeatureID }
join e in _dataSet.Employees.AsLive()
on i.AssignedTo equals e.EmployeeID
select new Issue
{
IssueID = i.IssueID,
ProductName = p.ProductName,
FeatureName = f.FeatureName,
Description = i.Description,
AssignedToID = e.EmployeeID,
AssignedToName = e.FullName
};
That view is indexed by the employee id field:
_bigView.Indexes.Add(x => x.AssignedToID);
so we can retrieve the items for a particular employee id at any time very fast.
Moreover, we can create a live view of that data given an employee id value (which is comboAssignedTo.SelectedIndex in this demo), simply by creating a view over the big view:
from i in _bigView where i.AssignedToID == comboAssignedTo.SelectedIndex select i;