Up Add a named query

Add the default named query

Last modified on December 05, 2012 16:47

The default named query is a server-side query method that represents the entire EntitySet for a given entity type.  In this topic we discuss what a default named query is and how to write one.

The default named query is a server-side query method that represents the entire EntitySet for a given entity type.  For example, the EntitySet for the Customer entity type is called “Customers”. When a client submits a query that is rooted in the “Customers” EntitySet and you’ve written a named query method for that EntitySet, such as GetCustomers, then that method is the default named query.

Conceptually the default named query means “return to the client every instance of this type.” You write this method on the server. Whether it in fact returns every instance of the type is up to you.

The following discussion begins with a client query for Customers when there is no named query. It identifies the root query to which you add filters, ordering, grouping, and selection. Then it shows two examples of a default named query that DevForce EntityServer substitutes for the root query when processing a query from the client.

A separate topic explains how to write a specialized named queries that can address a narrower business purpose.

A simple client query

Here is a query that you might write on the client to retrieve all customers in the database.

query = myEntityManager.Customers; // uses the generated query property
query = myEntityManager.Customers ' uses the generated query property

Customers is a factory property that returns an EntityQuery for Customer entities. The DevForce code generator added this property to the model’s custom EntityManager for your convenience. When you examine the generated code file, you’ll see that it was implemented as follows:

query = new EntityQuery<Customer>("Customers", myEntityManager);
query = New EntityQuery(Of Customer)("Customers", myEntityManager)

Locate the EntityQueries region of the generated entity class file.

The string, “Customers”, is the name of the EntitySet associated with the Customer entity type. DevForce interprets this to mean “the unrestricted set of all customers”.

The query is also associated with a particular EntityManager, the one in the myEntityManager variable. That means the query can be executed directly by writing something like this

query.ExecuteAsync(queryCallBack); // get the results in the callback
query.ExecuteAsync(queryCallBack) ' get the results in the callback

We don’t have to specify the EntityManager when we create the query. We might want to re-use the query with different EntityManager instances in which case we can write.

query = new EntityQuery<Customer>("Customers");
em1.ExecuteQueryAsync(query, queryCallback);
em2.ExecuteQueryAsync(query, queryCallback);
query = New EntityQuery(Of Customer)("Customers")
em1.ExecuteQueryAsync(query, queryCallback)
em2.ExecuteQueryAsync(query, queryCallback)

Query Root

The client query examples we’ve seen so far return all customers in the database. Of course you can restrict the query further by adding Where clauses such as one that returns only customers beginning with the letter “B”.

query-B-Customers = new EntityQuery<Customer>("Customers")
                        .Where(c => c.StartsWith("B");
myEntityManager.ExecuteQueryAsync(query-B-Customers, queryCallback);
query-B-Customers = New EntityQuery(Of Customer)("Customers").Where(Function(c) c.StartsWith("B")
myEntityManager.ExecuteQueryAsync(query-B-Customers, queryCallback)

Both the original query and this filtered query begin with the same root, the EntityQuery("Customers"). This is the root of the query no matter what LINQ clauses are added, no matter what types are ultimately returned.

“Customers” is the name of the EntitySet that includes all Customer entities in the database. It defines the default query.

Keep your eye on the root and the EntitySet name. If you create a specialized named query (as described elsewhere), you will create a different root that uses a different EntitySet name.

Add a do-nothing named query

Now let’s write a default named query method in a named query provider class.

public IQueryable<Customer> GetCustomers() {
 return new EntityQuery<Customer>();
Public Function GetCustomers() As IQueryable(Of Customer)
 Return New EntityQuery(Of Customer)()
End Function

Named queries are methods that return an IQueryable or an IEnumerable of an entity type. GetCustomers returns an IQueryable of Customer.

The definition of this query method should seem familiar. The returned value is almost exactly the same as the EntityQuery we wrote on the client. It’s only missing the EntitySet string (“Customers”). That’s OK because DevForce infers the “Customers” entity set name automatically.

It also lacks an EntityManager. That is to be expected on the server; the query will be used by a different EntityManager instance every time.

DevForce can tell that this is a query method for the Customer type because it has the right signature. It has no parameters and returns an IQueryable of Customer.  DevForce also determines that this is the default named query: the named query to use when the client sends a query specifying the “Customers” EntitySet

The EntityServer found this method by applying the DevForce query naming conventions during its search. It stripped off the “Get” from the method name, leaving the word “Customers” which matches the name of the Customer type’s EntitySet. You can use the Query 

Merging the named query with the original client query

Once DevForce finds the named query that matches the client’s query, it merges the two queries by copying the LINQ clauses of the client query to the output of the named query method. You can imagine DevForce doing something like the following with query-B-Customers:

// query-B-Customers = new EntityQuery<Customer>().Where(c => c.StartsWith("B");
mergedQuery = GetCustomers().Where(c => c.StartsWith("B");
' query-B-Customers = new EntityQuery<Customer>().Where(c => c.StartsWith("B");
mergedQuery = GetCustomers().Where(Function(c) c.StartsWith("B")

Then the EntityServer forwards the mergedQuery to the DevForce EntityServerQueryInterceptor or to your custom EntityServerQueryInterceptor

Add a do-something named query

The GetCustomers method we just wrote does exactly what DevForce would do anyway. There’s no point in writing a default named query unless it adds value.

You do not have to write a default named query.

But we can write a GetCustomers method that adds value such as this one:

public IQueryable<Customer> GetCustomers() {
   return new EntityQuery<Customer>().Include("Orders");
Public Function GetCustomers() As IQueryable(Of Customer)
  Return New EntityQuery(Of Customer)().Include("Orders")
End Function

Now only administrators can issue a query for Customers, thanks to the RequiresRoles attribute . The query also automatically includes the Order entities related to the Customers returned by the query.

The result returned to the client will be further restricted to Customers whose names begin with “B” because that criterion was copied over from the client's query-B-Customers query.

Let’s try a different implementation:

public IQueryable<Customer> GetCustomers() {
    var user = System.Threading.Thread.CurrentPrincipal as User;
   // only show the user’s own Customers
   return new EntityQuery<Customer>()
               .Where(c => c.UserID = user.ID);
Public Function GetCustomers() As IQueryable(Of Customer)
Dim user = TryCast(System.Threading.Thread.CurrentPrincipal, User)
  ' only show the user’s own Customers
  Return New EntityQuery(Of Customer)().Where(Function(c) c.UserID = user.ID)
End Function

In this one, the named query extracts the user from the server IPrincipal and limits the query to Customers that belong to the client user only. The EnsureValidUser method (not shown) throws a useful exception if the IPrincipal is not a proper User object.

After merging with the client query-B-Customers, the query results contain only those Customers whose names begin with “B” and

The client query does not change

You can add, modify, or delete the default named query without changing your client query. Client query syntax remains the same and the query is used the same way, with or without a default named query on the server. The presence, absence, or nature of the default named query is structurally and functionally transparent to the client.

Of course the default named query itself can change what entities are returned from the data store.  That’s why you wrote it. The named query may even reject the client query and throw an exception. These behaviors are up to you.

The default named query applies to the cache

A client query that is rooted in the default named query can be applied to the local entity cache.

If the query’s QueryStrategy is Optimized or DataSourceThenCache or CacheOnly, the query will include cached entities. If the EntityManager is disconnected, the query applies to the cache. The EntityManager can also remember the query in its query cache, potentially avoiding a redundant trip to the server when the query is executed a second time.

This is not true for specialized named queries described elsewhere. They are always executed as DataSourceOnly queries and are never remembered in the query cache. They always fail if executed offline. The reason for this difference is explained in the topic on specialized named queries.

Only one default named query

You don’t have to write a default named query. If you do, there can be only one named query method per entity type. There can be only one named query method associated with an entity type’s EntitySet and that one method must not take parameters.

Another topic covers parameterized named queries. The default named query may not have parameters.

Tags: Query
Created by DevForce on April 22, 2011 14:53

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