How to Perform Operations Lazily

Load entities

• Load loads a document entity from the database into the session.
  Loading entities can be executed lazily.

Lazy<Employee> lazyEmployee = session
     // Add a call to Lazily 
    .Advanced.Lazily
     // Document will Not be loaded from the database here, no server call is made
    .Load<Employee>("employees/1-A");

Employee employee = lazyEmployee.Value; // 'Load' operation is executed here
// The employee entity is now loaded & tracked by the session

Load entities with include

• Load with include loads both the document and the specified related document.
  Loading entities with include can be executed lazily.

Lazy<Product> lazyProduct = session
     // Add a call to Lazily 
    .Advanced.Lazily
     // Request to include the related Supplier document
     // Documents will Not be loaded from the database here, no server call is made
    .Include<Product>(x => x.SupplierId) 
    .Load<Product>("products/1-A"); 

// 'Load with include' operation will be executed here
// Both documents will be retrieved from the database
Product product = lazyProduct.Value;
// The product entity is now loaded & tracked by the session

// Access the related document, no additional server call is made
Supplier supplier = session.Load<Supplier>(product.SupplierId);
// The supplier entity is now also loaded & tracked by the session

public class Product
{
    public string Id { get; set; }
    public string Name { get; set; }
    public string SupplierId { get; set; } // The related document ID
}

Load entities starting with

• LoadStartingWith loads entities whose ID starts with the specified prefix.
  Loading entities with a common prefix can be executed lazily.

Lazy<Dictionary<string, Employee>> lazyEmployees = session
     // Add a call to Lazily 
    .Advanced.Lazily
     // Request to load entities whose ID starts with 'employees/'
     // Documents will Not be loaded from the database here, no server call is made
    .LoadStartingWith<Employee>("employees/");

var employees = lazyEmployees.Value; // 'Load' operation is executed here
// The employee entities are now loaded & tracked by the session

Conditional load

• ConditionalLoad logic is:
  • If the entity is already loaded to the session:
    no server call is made, the tracked entity is returned.
  • If the entity is Not already loaded to the session:
    the document will be loaded from the server only if the change-vector provided to the method is older than the one in the server (i.e. if the document in the server is newer).
  • Loading entities conditionally can be executed lazily.

// Create document and get its change-vector:
string changeVector; 
using (var session1 = store.OpenSession())
{
    Employee employee = new Employee();
    session1.Store(employee, "employees/1-A");
    session1.SaveChanges();
    
    // Get the tracked entity change-vector
    changeVector = session1.Advanced.GetChangeVectorFor(employee);
}

// Conditionally lazy-load the document:
using (var session2 = store.OpenSession())
{
    var lazyEmployee = session2
         // Add a call to Lazily 
        .Advanced.Lazily
         // Document will Not be loaded from the database here, no server call is made
        .ConditionalLoad<Employee>("employees/1-A", changeVector);

    var loadedItem = lazyEmployee.Value; // 'ConditionalLoad' operation is executed here
    Employee employee = loadedItem.Entity;
    
    // If ConditionalLoad has actually fetched the document from the server (logic described above)
    // then the employee entity is now loaded & tracked by the session
    
}

Run query

• A Query can be executing a lazily.
  Learn more about running queries lazily in lazy queries.

// Define a lazy query:
Lazy<IEnumerable<Employee>> lazyEmployees = session
    .Query<Employee>()
    .Where( x => x.FirstName == "John")
     // Add a call to Lazily, the query will Not be executed here 
    .Lazily();

IEnumerable<Employee> employees = lazyEmployees.Value; // Query is executed here

// Note: Since query results are not projected,
// then the resulting employee entities will be tracked by the session.

Get revisions

• All methods for getting revisions and their metadata can be executed lazily.

var lazyRevisions = session
     // Add a call to Lazily 
    .Advanced.Revisions.Lazily
     // Revisions will Not be fetched here, no server call is made
    .GetFor<Employee>("employees/1-A");

     // Usage is the same for the other get revisions methods:
     // .Get()
     // .GetMetadataFor()

List<Employee> revisions = lazyRevisions.Value; // Getting revisions is executed here

Get compare-exchange value

• Getting compare-exchange values can be executed lazily.

using (var session =
       store.OpenSession(new SessionOptions { TransactionMode = TransactionMode.ClusterWide }))
{
    // Create compare-exchange value:
    session.Advanced.ClusterTransaction
        .CreateCompareExchangeValue(key: "someKey", value: "someValue");
    session.SaveChanges();

    // Get the compare-exchange value lazily: 
    var lazyCmpXchg = session
        // Add a call to Lazily 
        .Advanced.ClusterTransaction.Lazily
        // Compare-exchange values will Not be fetched here, no server call is made
        .GetCompareExchangeValue<string>("someKey");
    
    // Usage is the same for the other method:
    // .GetCompareExchangeValues()

    CompareExchangeValue<string> cmpXchgValue =
        lazyCmpXchg.Value; // Getting compare-exchange value is executed here
}