Session: Loading Entities
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There are several methods that allow users to load documents from the database and convert them to entities.
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This article covers the following methods:
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For loading entities lazily see perform requests lazily.
Load
The most basic way to load a single entity is to use session's load()
method.
await session.load(id, [documentType]);
Parameters | ||
---|---|---|
id | string | Identifier of a document that will be loaded. |
documentType | function | A class constructor used for reviving the results' entities |
Return Value | |
---|---|
Promise<object> |
A Promise returning object or null if a document with a given ID does not exist. |
Example
const employee = await session.load("employees/1");
Note
In 4.x RavenDB, only string identifiers are supported. If you are upgrading from 3.x, this is a major change, because in 3.x non-string identifiers are supported.
Load with Includes
When there is a relationship between documents, those documents can be loaded in a single request call using the include()
and load()
methods.
session.include(path);
Parameters | ||
---|---|---|
path | string | Field path in documents in which the server should look for 'referenced' documents. |
Return Value | |
---|---|
object{load()} |
The include() method by itself does not materialize any requests but returns loader containing methods such as load() . |
Example I
We can use this code to also load an employee which made the order.
// loading 'products/1'
// including document found in 'supplier' property
const product = await session
.include("supplier")
.load("products/1");
const supplier = await session.load(product.supplier); // this will *not* make a server call
Load - multiple entities
To load multiple entities at once, use one of the following ways to call load()
.
await session.load(idsArray, [documentType]);
await session.load(idsArray, [options]);
Parameters | ||
---|---|---|
idsArray | string[] | Multiple document identifiers to load |
documentType | function | A class constructor used for reviving the results' entities |
options | string | Options with the following properties |
documentType | function | A class construcor used for reviving the results' entities |
includes | string[] | Field paths in documents in which the server should look for 'referenced' documents. |
Return Value | |
---|---|
Promise<{ [id]: object }> |
A Promise resolving to an object mapping document identifiers to object or null if a document with given ID doesn't exist |
const employees = await session.load(
["employees/1", "employees/2", "employees/3"]);
// {
// "employees/1": { ... },
// "employees/2": { ... }
// "employees/3": { ... }
// }
LoadStartingWith
To load multiple entities that contain a common prefix, use the loadStartingWith()
method from the advanced
session operations.
await session.advanced.loadStartingWith(idPrefix, [options]);
await session.advanced.loadStartingWithIntoStream(idPrefix, output, [options]);
Parameters | ||
---|---|---|
idPrefix | string | prefix for which the documents should be returned |
options | string | Options with the following properties |
matches | string | pipe ('|') separated values for which document IDs (after 'idPrefix') should be matched ('?' any single character, '*' any characters) |
start | number | number of documents that should be skipped |
pageSize | number | maximum number of documents that will be retrieved |
exclude | string | pipe ('|') separated values for which document IDs (after 'idPrefix') should not be matched ('?' any single character, '*' any characters) |
skipAfter | string | skip document fetching until given ID is found and return documents after that ID (default: null ) |
documentType | function | A class constructor used for reviving the results' entities |
Return Value | |
---|---|
Promise<object[]> |
A Promise resolving to an array of entities matching given parameters |
Example I
// return up to 128 entities with Id that starts with 'employees'
const result = await session
.advanced
.loadStartingWith("employees/", {
start: 0,
pageSize: 128
});
Example II
// return up to 128 entities with Id that starts with 'employees/'
// and rest of the key begins with "1" or "2" e.g. employees/10, employees/25
const result = await session
.advanced
.loadStartingWith("employees/", {
matches: "1*|2*",
start: 0,
pageSize: 128
});
ConditionalLoad
This method can be used to check whether a document has been modified since the last time its change vector was recorded, so that the cost of loading it can be saved if it has not been modified.
The conditionalLoad
method takes a document's change vector.
If the entity is tracked by the session, this method returns the entity. If the entity
is not tracked, it checks if the provided change vector matches the document's
current change vector on the server side. If they match, the entity is not loaded.
If the change vectors do not match, the document is loaded.
The method is accessible from the session.Advanced
operations.
await session.advanced.conditionalLoad(id, changeVector, clazz);
Parameter | Type | Description |
---|---|---|
id | string |
The identifier of a document to be loaded. |
changeVector | string |
The change vector you want to compare with the server-side change vector. If the change vectors match, the document is not loaded. |
Return Type | Description |
---|---|
ValueTuple (object, changeVector) |
If the given change vector and the server side change vector do not match, the method returns the requested entity and its current change vector. If the change vectors match, the method returns default as the entity, and the current change vector.If the specified document, the method returns only default without a change vector. |
Example
const session = store.openSession();
const user = new User("Bob");
await session.store(user, "users/1");
await session.saveChanges();
const changeVector = session.advanced.getChangeVectorFor(user);
{
// New session which does not track our User entity
// The given change vector matches
// the server-side change vector
// Does not load the document
const session = store.openSession();
const result1 = await session.advanced
.conditionalLoad("users/1", changeVector, User);
// Modify the document
user.name = "Bob Smith";
await session.store(user);
await session.saveChanges();
// Change vectors do not match
// Loads the document
const result2 = await session.advanced
.conditionalLoad("users/1", changeVector, User);
Stream
Entities can be streamed from the server using the stream()
method from the advanced
session operations.
// stream query results
await session.stream(query, [statsCallback]);
// stream documents with ID starting with
await session.stream(idPrefix, [options]);
Parameters | ||
---|---|---|
idPrefix | string | prefix for which the documents should be returned |
query | query object | a query obtained from a call to session.query() or session.advanced.rawQuery() |
options | string | Options with the following properties |
startsWith | string | prefix for which documents should be streamed |
matches | string | pipe ('|') separated values for which document IDs should be matched ('?' any single character, '*' any characters) |
start | number | number of documents that should be skipped |
pageSize | number | maximum number of documents that will be retrieved |
skipAfter | string | skip document fetching until a given ID is found and returns documents after that ID (default: null ) |
documentType | function | A class constructor used for reviving the results' entities |
statsCallback | function | callback returning information about the streaming query (amount of results, which index was queried, etc.) |
Return Value | |
---|---|
Promise<Readable> |
A Promise resolving to readable stream with query results |
Example I
Stream documents for a ID prefix:
// stream() returns a Node.js Readable
const stream = await session.advanced.stream("employees/");
stream.on("data", data => {
// Employee { name: 'Anna', id: 'employees/1-A' }
});
stream.on("error", err => {
// handle errors
});
stream.on("end", () => {
// stream ended
});
Example 2
Fetch documents for a ID prefix directly into a writable stream:
const employeesFile = fs.createWriteStream("employees.json");
await session.advanced.loadStartingWithIntoStream("employees/", employeesFile);
Information
Entities loaded using stream()
will be transient (not attached to session).
IsLoaded
Use the isLoaded
method from the advanced
session operations
To check if an entity is attached to a session (e.g. because it's been
previously loaded).
isLoaded
checks if an attempt to load a document has been already made
during the current session, and returns true
even if such an attemp was
made and failed.
If, for example, the load
method was used to load employees/3
during
this session and failed because the document has been previously deleted,
isLoaded
will still return true
for employees/3
for the remainder
of the session just because of the attempt to load it.
session.advanced.isLoaded(id);
Parameters | ||
---|---|---|
id | string | Entity ID for which the check should be performed. |
Return Value | |
---|---|
boolean | Indicates if an entity with a given ID is loaded. |
Example
session.advanced.isLoaded("employees/1"); // false
const employee = await session.load("employees/1");
session.advanced.isLoaded("employees/1"); // true
On entities loading, JS classes and the documentType parameter
Type information about the entity and its contents is by default stored in the document metadata. Based on that its types are revived when loaded from the server.
Entity type registration
In order to avoid passing documentType argument every time, you can register the type in the document conventions using the registerEntityType()
method before calling DocumentStore's initialize()
like so:
class Pet {
constructor(name) {
this.name = name;
}
}
class Person {
constructor(name, pet) {
this.name = name;
this.pet = pet;
}
}
documentStore.conventions.registerEntityType(Person);
documentStore.conventions.registerEntityType(Pet);
// ...
documentStore.initialize();
If you fail to do so, entities (and all subobjects) loaded from the server are going to be plain object literals and not instances of the original type they were stored with.