Fetching DataReaders and projections

Besides using Linq or QuerySpec, LLBLGen Pro has two other ways of fetching a resultset of fields: as an open IDataReader object and as a projection. This section discusses both and offers examples for each of them, either using a stored procedure or a query build using entity fields.

Fetching a resultset as an open IDataReader is considered an advanced feature and should be used with care: an open IDataReader object represents an open cursor to data on a connected RDBMS over an open connection. This means that passing the IDataReader around in your application is not recommended. Instead use the IDataReader in the method where you also called the fetch logic to create it and immediately after that make sure the IDataReader gets closed and disposed. This way you're sure you'll free up resources early.

To understand projections better, it's recommended to first read the section about fetching an open IDataReader. Another section describing projections, but then related to an entity view object, is Generated code - using the EntityView2 class.

Although QuerySpec and Linq offer ways to fetch projections into objects, this section primarily discusses projections using the low-level API. QuerySpec also supports fetching a projection as IDataReader. An example of this is given below in the section about fetching a dynamic list as IDataReader.

Fetching a resultset as an open IDataReader

To fetch a resultset as an open IDataReader, you call one of the overloads of FetchDataReader, a method of DataAccessAdapter. There are two ways to use the FetchDataReader method: by supplying a ready to use IRetrievalQuery or by specifying a fields list, and various other elements which are required for creating a new query by the Dynamic Query Engine (DQE).

The first option, the IRetrievalQuery option, can be used to fetch a retrieval stored procedure as an open IDataReader, by using the RetrievalProcedures.GetStoredProcedureNameCallAsQuery() method of the particular stored procedure call. This is a generated method, one for every retrieval stored procedure call known in the LLBLGen Pro project.

FetchDataReader accepts also a parameter called CommandBehavior. This parameter is very important as it controls the behavior the datareader should perform when the datareader is closed. It's only required to specify a behavior different than CloseConnection if the fetch is inside a transaction and the connection has to stay open after the datareader has been closed.

It's possible to construct your own IRetrievalQuery object with your own SQL, by instantiating a new RetrievalQuery object. However in general, it's recommended to use the FetchDataReader overloads which accept a fieldslist and other elements and let LLBLGen Pro generate the query for you.

Fetching a Retrieval Stored Procedure as an IDataReader

An example of calling a procedure and receive a datareader from it is enlisted below. It calls the Northwind stored procedure CustOrdersOrders which returns a single resultset with 4 fields. The example simply prints the output on the console.

C#VB.NET

using( DataAccessAdapter adapter = new DataAccessAdapter() )
{
    IDataReader reader = adapter.FetchDataReader( 
        RetrievalProcedures.GetCustOrdersOrdersCallAsQuery( "CHOPS" ), 
        CommandBehavior.CloseConnection );
    while( reader.Read() )
    {
        Console.WriteLine( "Row: {0} | {1} | {2} | {3} |", reader.GetValue( 0 ), 
            reader.GetValue( 1 ), reader.GetValue( 2 ), reader.GetValue( 3 ) );
    }
    // close reader, will also close connection
    reader.Close();
}

Using adapter As New DataAccessAdapter()
    Dim reader as IDataReader = adapter.FetchDataReader( _
        RetrievalProcedures.GetCustOrdersOrdersCallAsQuery( "CHOPS" ), _
        CommandBehavior.CloseConnection )
    While reader.Read()
        Console.WriteLine( "Row: {0} | {1} | {2} | {3} |", reader.GetValue( 0 ), _
            reader.GetValue( 1 ), reader.GetValue( 2 ), reader.GetValue( 3 ) )
    End While
    ' close reader, will also close connection
    reader.Close()
End Using

Fetching a Dynamic List as an IDataReader

An example of a dynamic list which is used to receive a datareader from it is enlisted below. The example simply prints the output on the console.

Low-level API, C#Low-level API, VB.NETQuerySpec, C#QuerySpec, VB.NET

using( DataAccessAdapter adapter = new DataAccessAdapter() )
{
    ResultsetFields fields = new ResultsetFields( 3 );
    // simply set the fields in the indexes, which will use the field name for the column name
    fields[0] = CustomerFields.CustomerId;
    fields[1] = CustomerFields.CompanyName;
    fields[2] = OrderFields.OrderId;
    RelationPredicateBucket filter = new RelationPredicateBucket(CustomerFields.Country.Equal("Germany"));
    filter.Relations.Add(CustomerEntity.Relations.OrderEntityUsingCustomerId);
    IDataReader reader = adapter.FetchDataReader( fields, filter, CommandBehavior.CloseConnection, 0, true );
    while( reader.Read() )
    {
        Console.WriteLine( "Row: {0} | {1} | {2} |", 
            reader.GetValue( 0 ), reader.GetValue( 1 ), reader.GetValue( 2 ) );
    }
    reader.Close();
}

Using adapter As New DataAccessAdapter()
    Dim fields As New ResultsetFields( 3 )
    ' simply set the fields in the indexes, which will use the field name for the column name
    fields(0) = CustomerFields.CustomerId
    fields(1) = CustomerFields.CompanyName
    fields(2) = OrderFields.OrderId
    Dim filter As New RelationPredicateBucket(CustomerFields.Country.Equal("Germany"))
    filter.Relations.Add(CustomerEntity.Relations.OrderEntityUsingCustomerId)
    Dim reader As IDataReader = adapter.FetchDataReader( fields, filter, CommandBehavior.CloseConnection, 0, True )
    While reader.Read()
        Console.WriteLine( "Row: {0} | {1} | {2} |", _
            reader.GetValue( 0 ), reader.GetValue( 1 ), reader.GetValue( 2 ) )
    End While
    reader.Close()
End Using

using(var adapter = new DataAccessAdapter())
{
    var qf = new QueryFactory();
    var q = qf.Create()
                .Select(CustomerFields.CustomerId, CustomerFields.CompanyName, 
                        OrderFields.OrderId)
                .From(qf.Customer.InnerJoin(qf.Order)
                        .On(CustomerFields.CustomerId == OrderFields.CustomerId))
                .Where(CustomerFields.Country == "Germany");

    var reader = adapter.FetchAsDataReader(q, CommandBehavior.CloseConnection);
    while(reader.Read())
    {
        Console.WriteLine("Row: {0} | {1} | {2} |",
        reader.GetValue(0), reader.GetValue(1), reader.GetValue(2));
    }
    reader.Close();
}

Using adapter As New DataAccessAdapter()
    Dim qf As New QueryFactory()
    Dim q = qf.Create() _
                .Select(CustomerFields.CustomerId, CustomerFields.CompanyName, 
                        OrderFields.OrderId) _
                .From(qf.Customer.InnerJoin(qf.Order)
                        .On(CustomerFields.CustomerId = OrderFields.CustomerId)) _
                .Where(CustomerFields.Country = "Germany")

    Dim reader = adapter.FetchAsDataReader(q, CommandBehavior.CloseConnection)
    While reader.Read()
        Console.WriteLine( "Row: {0} | {1} | {2} |", _
            reader.GetValue( 0 ), reader.GetValue( 1 ), reader.GetValue( 2 ) )
    End While
    reader.Close()
End Using

Resultset projections

In the previous section we've seen that a query could be fetched as an open IDataReader, where the query could be an IRetrievalQuery object containing a stored procedure call, or a dynamic formulated query from fields, a filter and other elements you might want to use in the query. It is then up to you what to do with the IDataReader.

It's likely you'll want to project the data available to you through the IDataReader object onto a data-structure. Projecting a resultset is a term from the relational algebra, the Wikipedia has a formal explanation of it: Projection (relational algebra) (opens in a new window). It comes down to the fact that you create a new set of data from an existing set of data. The existing set of data is the resultset you want to project. The new set is the projection result.

LLBLGen Pro offers two different projection mechanisms: projecting an EntityView2<T> (see: Generated code - using the EntityView2 class) and projecting a fetched resultset, which is discussed here. Both mechanisms are roughly the same, only the source data origin differs and the used interface implemented by the used projection engine.

The projections of entity view data are a little more advanced because it's possible to execute in-memory filters on the entity object itself to make a selection which field to project.

For projections of EntityView2<T> data, EntityPropertyProjector objects are used, for projections of resultset data, the more simpler DataValueProjector objects are used. Their meaning is roughly the same, so if you're familiar with EntityView2<T> projections, you'll directly understand the examples below using DataValueProjector objects. As the projection engine interfaces required for both mechanisms are fairly similar, the shipped projection engines thereby can be used for both mechanisms.

Resultset projections are done by an IGeneralDataProjector implementation. IGeneralDataProjector allows an object[] object of values to be projected onto new instances of whatever class is supported by the IGeneralDataProjector implementation, for example new entities or a DataRow in a DataTable. Which values in the object[] object are projected onto which properties of the target element, created by the IGeneralDataProjector implementation, is specified by the specified set of IDataValueProjector implementations passed in.

In Adapter, the DataAccessAdapter class has a method called FetchProjection with various overloads. This method produces the projection of the resultset defined by the input parameters (similar to the FetchDataReader method) or the resultset passed in in the form of an open IDataReader object.

By which projection engine the projection is performed as well which data is projected is passed in as well. FetchProjection doesn't return a value, the result is in the projection engine object. This method has similar overloads as FetchDataReader, though it doesn't accept a CommandBehavior: if a connection is open, it leaves it open, if no connection is open, it creates one and closes one afterwards.

Projecting Stored Procedure resultset onto entity collection

For this stored procedure projection example, the following stored procedure is used:

CREATE  procedure pr_CustomersAndOrdersOnCountry
    @country VARCHAR(50)
AS
SELECT * FROM Customers WHERE Country = @country
SELECT * FROM Orders WHERE CustomerID IN
(
    SELECT CustomerID FROM Customers WHERE Country = @country
)

which is a SQL Server stored procedure and which returns 2 resultsets: the first is all customers filtered on a given Country, and the second is all orders of those filtered customers.

The stored procedure is fetched as an open IDataReader and both resultsets are projected onto entity collections: the first resultset on an EntityCollection object with CustomerEntity instances and the second on an EntityCollection of OrderEntity instances. The stored procedure uses a wildcard select list. This is for simplicity.

C#VB.NET

var customers = new EntityCollection<CustomerEntity>();
var orders = new EntityCollection<OrderEntity>();
using(var query = RetrievalProcedures.GetCustomersAndOrdersOnCountryCallAsQuery("Germany"))
{
    using(var adapter = new DataAccessAdapter())
    {
        using(var reader = adapter.FetchDataReader(query, CommandBehavior.CloseConnection))
        {
            // first resultset: Customers.
            var valueProjectors = new List<IDataValueProjector>();
            // project value on index 0 in resultset row onto CustomerId
            valueProjectors.Add( new DataValueProjector(CustomerFieldIndex.CustomerId.ToString(), 0, typeof(string)));
            // project value on index 1 in resultset row onto CompanyName
            valueProjectors.Add( new DataValueProjector(CustomerFieldIndex.CompanyName.ToString(), 1, typeof(string)));
            // resultset contains more rows, we just project those 2. The rest is trivial.
            var projector = new DataProjectorToIEntityCollection2(customers);
            adapter.FetchProjection(valueProjectors, projector, reader );

            // second resultset: Orders. 
            valueProjectors = new ArrayList();
            valueProjectors.Add(new DataValueProjector(OrderFieldIndex.OrderId.ToString(), 0, typeof(int)));
            valueProjectors.Add(new DataValueProjector(OrderFieldIndex.CustomerId.ToString(), 1, typeof(string)));
            valueProjectors.Add(new DataValueProjector(OrderFieldIndex.OrderDate.ToString(), 3, typeof(DateTime)));
            // switch to the next resultset in the datareader
            reader.NextResult();
            projector = new DataProjectorToIEntityCollection2(orders);
            adapter.FetchProjection(valueProjectors, projector, reader);
            reader.Close();
        }
    }
}

Dim customers As New EntityCollection(Of CustomerEntity)()
Dim orders As New EntityCollection(Of OrderEntity)()
Using query As IRetrievalQuery = RetrievalProcedures.GetCustomersAndOrdersOnCountryCallAsQuery( "Germany" )
    Using adapter As New DataAccessAdapter()
        Using reader As IDataReader = adapter.FetchDataReader(query, CommandBehavior.CloseConnection))
            ' first resultset: Customers.
            Dim valueProjectors As New List(Of IDataValueProjector)()
            ' project value on index 0 in resultset row onto CustomerId
            valueProjectors.Add( New DataValueProjector( CustomerFieldIndex.CustomerId.ToString(), 0, GetType( Sring ) ) )
            ' project value on index 1 in resultset row onto CompanyName
            valueProjectors.Add( New DataValueProjector( CustomerFieldIndex.CompanyName.ToString(), 1, GetType( String ) ) )
            ' resultset contains more rows, we just project those 2. The rest is trivial.
            Dim projector As New DataProjectorToIEntityCollection2( customers )
            adapter.FetchProjection( valueProjectors, projector, reader )

            ' second resultset: Orders. 
            valueProjectors = New ArrayList()
            valueProjectors.Add( New DataValueProjector( OrderFieldIndex.OrderId.ToString(), 0, GetType( Integer ) ) )
            valueProjectors.Add( New DataValueProjector( OrderFieldIndex.CustomerId.ToString(), 1, GetType( String ) ) )
            valueProjectors.Add( New DataValueProjector( OrderFieldIndex.OrderDate.ToString(), 3, GetType( DateTime ) ) )
            ' switch to the next resultset in the datareader
            reader.NextResult()
            projector = New DataProjectorToIEntityCollection2( orders )
            adapter.FetchProjection( valueProjectors, projector, reader )
            reader.Close()
        End Using
    End Using
End Using

Projecting Dynamic List resultset onto custom classes

We can go one step further and create a fetch of a dynamic list and fill a list of custom class instances, for example for transportation by a Webservice and you want lightweight Data Transfer Objects (DTO). For clarity, the Linq and QuerySpec alternatives are given as well.

Low-level API, C#QuerySpec, C#Linq, C#

var customClasses = new List<CustomCustomer>();
var fields = new ResultsetFields( 4 );
fields[0] = CustomerFields.City;
fields[1] = CustomerFields.CompanyName;
fields[2] = CustomerFields.CustomerId;
fields[3] = CustomerFields.Country;

var projector = 
    new DataProjectorToCustomClass<CustomCustomer>( customClasses );

// Define the projections of the fields.    
var valueProjectors = new List<IDataValueProjector>();
valueProjectors.Add( new DataValueProjector( "City", 0, typeof( string ) ) );
valueProjectors.Add( new DataValueProjector( "CompanyName", 1, typeof( string ) ) );
valueProjectors.Add( new DataValueProjector( "CustomerID", 2, typeof( string ) ) );
valueProjectors.Add( new DataValueProjector( "Country", 3, typeof( string ) ) );

// perform the fetch combined with the projection.
using( DataAccessAdapter adapter = new DataAccessAdapter() )
{
    adapter.FetchProjection( valueProjectors, projector, fields, null, 0, true );
}

List<CustomCustomer> customClasses;
using(var adapter = new DataAccessAdapter())
{
    var qf = new QueryFactory();
    var q = qf.Customer
        .Select(() => new CustomCustomer()
        {
            City = CustomerFields.City.ToValue<string>(),
            CompanyName = CustomerFields.CompanyName.ToValue<string>(),
            Country = CustomerFields.Country.ToValue<string>(),
            CustomerID = CustomerFields.CustomerId.ToValue<string>()
        });
    customClasses = adapter.FetchQuery(q);
}

List<CustomCustomer> customClasses;
using(var adapter = new DataAccessAdapter())
{
    var metaData = new LinqMetaData(adapter);
    var q = from c in metaData.Customer
        select new CustomCustomer()
        {
            CustomerID = c.CustomerId,
            CompanyName = c.CompanyName,
            Country = c.Country,
            City = c.City
        };
    customClasses = q.ToList();
}

Where the custom class is:

public class CustomCustomer
{
    private string _customerID, _companyName, _city, _country;

    public CustomCustomer()
    {
        _city = string.Empty;
        _companyName = string.Empty;
        _customerID = string.Empty;
        _country = string.Empty;
    }

    public string CustomerID
    {
        get { return _customerID; }
        set { _customerID = value; }
    }

    public string City
    {
        get { return _city; }
        set { _city = value; }
    }

    public string CompanyName
    {
        get { return _companyName; }
        set { _companyName = value; }
    }

    public string Country
    {
        get { return _country; }
        set { _country = value; }
    }
}