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Parent/Child relationships

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Parent/Child relationships

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Prior to Elasticsearch 6.x you could have multiple types in a single index. Through the special _parent field mapping of a given type, one could create 1 to N relationships of parent ⇒ children documents. This worked because when indexing children, you passed a _parent id which acted as the routing key, ensuring a parent, its children and any ancestors all lived on the same shard.

Starting with 6.x indices, multiple types are no longer suppported in a single index. One reason for this is that if for instance two types have the same name property, this property needed to be mapped exactly the same for both types, but all the APIs act as if you can map them individually, often causing confusion. Essentially, _type always acted as a discriminating field within an index but was often explained as being more special than this.

So how do you create a parent join, now that indices no longer allow you store different types in the same index and therefor also not on the same shard?

Parent And Child example

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In the following contrived example we create two .NET types called MyParent and MyChild both extending from MyDocument. There is no requirement that the parent and child .NET types are related at all. The types could be plain POCO’s or MyChild could be a subclass of MyParent. The only requirement is that they have a property where the property type is JoinField.

As per Elasticsearch’s constraints you should only have a single property of type JoinField on your POCO.

public abstract class MyDocument
{
    public int Id { get; set; }
    public JoinField MyJoinField { get; set; }
}

public class MyParent : MyDocument
{
    [Text]
    public string ParentProperty { get; set; }
}

public class MyChild : MyDocument
{
    [Text]
    public string ChildProperty { get; set; }
}

Parent And Child mapping

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In the following example we setup our client and give our types prefered index and type names. Starting with NEST 6.x we can also give a type a preferred RelationName as can be seen on the DefaultMappingFor<MyParent>.

Also note that we give MyChild and MyParent the same default doc type name to make sure they end up in the same index under the same type.

var connectionPool = new SingleNodeConnectionPool(new Uri("http://localhost:9200"));
var connectionSettings = new ConnectionSettings(connectionPool, new InMemoryConnection()) 
    .DefaultMappingFor<MyDocument>(m => m.IndexName("index").TypeName("doc"))
    .DefaultMappingFor<MyChild>(m => m.IndexName("index").TypeName("doc"))
    .DefaultMappingFor<MyParent>(m => m.IndexName("index").TypeName("doc").RelationName("parent"));

var client = new ElasticClient(connectionSettings);

for the purposes of this example, an in memory connection is used which doesn’t actually send a request. In your application, you’d use the default connection or your own implementation that actually sends a request.

With the ConnectionSettings set up, we can proceed to map MyParent and MyChild as part of the create index request.

var createIndexResponse = client.CreateIndex("index", c => c
    .Index<MyDocument>()
    .Mappings(ms => ms
        .Map<MyDocument>(m => m
            .RoutingField(r => r.Required()) 
            .AutoMap<MyParent>() 
            .AutoMap<MyChild>() 
            .Properties(props => props
                .Join(j => j 
                    .Name(p => p.MyJoinField)
                    .Relations(r => r
                        .Join<MyParent, MyChild>()
                    )
                )
            )
        )
    )
);

	recommended to make the routing field mandatory so you can not accidentally forget
	Map all of the `MyParent` properties
	Map all of the `MyChild` properties
	Additionally map the `JoinField` since it is not automatically mapped by `AutoMap()`

We call AutoMap() for both types to discover properties of both .NET types. AutoMap() won’t automatically setup the join field mapping though because NEST can not infer all the Relations that are required by your domain.

In this case we setup MyChild to be child of MyParent. .Join() has many overloads so be sure to check them out if you need to map not one but multiple children.

{
  "mappings": {
    "doc": {
      "_routing": {
        "required": true
      },
      "properties": {
        "parentProperty": {
          "type": "text"
        },
        "childProperty": {
          "type": "text"
        },
        "id": {
          "type": "integer"
        },
        "myJoinField": {
          "type": "join",
          "relations": {
            "parent": "mychild"
          }
        }
      }
    }
  }
}

Note how MyParent's relation name is parent because of the mapping on connection settings. This also comes in handy later when doing strongly typed has_child and has_parent queries.

Indexing parents or children

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Now that we have our join field mapping set up on the index, we can proceed to index parent and child documents.

To mark a document with the relation name of the parent MyParent all of the following three ways are equivalent.

In the first we explicitly call JoinField.Root to mark this document as the root of a parent child relationship namely that of MyParent. In the following examples we rely on implicit conversion from string and Type to do the same.

var parentDocument = new MyParent
{
    Id = 1,
    ParentProperty = "a parent prop",
    MyJoinField = JoinField.Root<MyParent>()
};

parentDocument = new MyParent
{
    Id = 1,
    ParentProperty = "a parent prop",
    MyJoinField = typeof(MyParent) 
};

parentDocument = new MyParent
{
    Id = 1,
    ParentProperty = "a parent prop",
    MyJoinField = "myparent" 
};
var indexParent = client.IndexDocument<MyDocument>(parentDocument);

	this lets the join data type know this is a root document of type `myparent`
	this lets the join data type know this is a root document of type `myparent`

{
  "id": 1,
  "parentProperty": "a parent prop",
  "myJoinField": "myparent"
}

Linking the child document to its parent follows a similar pattern. Here we create a link by inferring the id from our parent instance parentDocument

var indexChild = client.IndexDocument<MyDocument>(new MyChild
{
    MyJoinField = JoinField.Link<MyChild, MyParent>(parentDocument)
});

or here we are simply stating this document is of type mychild and should be linked to parent id 1 from parentDocument.

indexChild = client.IndexDocument<MyDocument>(new MyChild
{
    Id = 2,
    MyJoinField = JoinField.Link<MyChild>(1)
});

{
  "id": 2,
  "myJoinField": {
    "name": "mychild",
    "parent": "1"
  }
}

The mapping already links myparent as the parent type so we only need supply the parent id. In fact there are many ways to create join field:

Expect("myparent").WhenSerializing(JoinField.Root(typeof(MyParent)));
Expect("myparent").WhenSerializing(JoinField.Root(Relation<MyParent>()));
Expect("myparent").WhenSerializing(JoinField.Root<MyParent>());
Expect("myparent").WhenSerializing(JoinField.Root("myparent"));

var childLink = new { name = "mychild", parent = "1" };
Expect(childLink).WhenSerializing(JoinField.Link<MyChild>(1));
Expect(childLink).WhenSerializing(JoinField.Link<MyChild, MyParent>(parentDocument));
Expect(childLink).WhenSerializing(JoinField.Link("mychild", 1));
Expect(childLink).WhenSerializing(JoinField.Link(typeof(MyChild), 1));

Routing parent child documents

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A parent and all of it’s (grand)children still need to live on the same shard so you still need to take care of specifying routing.

In the past you would have to provide the parent id on the request using parent=<parentid> this was always an alias for routing and thus in Elasticsearch 6.x you need to provide routing=<parentid> instead.

NEST has a handy helper to infer the correct routing value given a document that is smart enough to find the join field and infer correct parent.

var infer = client.Infer;
var parent = new MyParent {Id = 1337, MyJoinField = JoinField.Root<MyParent>()};
infer.Routing(parent).Should().Be("1337");

var child = new MyChild {Id = 1338, MyJoinField = JoinField.Link<MyChild>(parentId: "1337")};
infer.Routing(child).Should().Be("1337");

child = new MyChild {Id = 1339, MyJoinField = JoinField.Link<MyChild, MyParent>(parent)};
infer.Routing(child).Should().Be("1337");

here we index parent and rather than fishing out the parent id by inspecting parent we just pass the instance to Routing which can infer the correct routing key based on the JoinField property on the instance

var indexResponse = client.Index(parent, i => i.Routing(Routing.From(parent)));
indexResponse.ApiCall.Uri.Query.Should().Contain("routing=1337");

The same goes for when we index a child, we can pass the instance directly to Routing and NEST will use the parent id already specified on child. Here we use the static import using static Nest.Infer and it’s Route() static method to create an instance of Routing

indexResponse = client.Index(child, i => i.Routing(Route(child)));
indexResponse.ApiCall.Uri.Query.Should().Contain("routing=1337");

Wouldn’t be handy if NEST does this automatically? It does!

indexResponse = client.IndexDocument(child);
indexResponse.ApiCall.Uri.Query.Should().Contain("routing=1337");

You can always override the default inferred routing though

indexResponse = client.Index(child, i => i.Routing("explicit"));
indexResponse.ApiCall.Uri.Query.Should().Contain("routing=explicit");

indexResponse = client.Index(child, i => i.Routing(null));
indexResponse.ApiCall.Uri.Query.Should().NotContain("routing");

This works for both the fluent and object initializer syntax

var indexRequest = new IndexRequest<MyChild>(child);
indexResponse = client.Index(indexRequest);
indexResponse.ApiCall.Uri.Query.Should().Contain("routing=1337");

Its important to note that the routing is resolved at request time, not instantation time here we update the child's JoinField after already creating the index request for child

child.MyJoinField = JoinField.Link<MyChild>(parentId: "something-else");
indexResponse = client.Index(indexRequest);
indexResponse.ApiCall.Uri.Query.Should().Contain("routing=something-else");

If you use multiple levels of parent and child relations e.g A => B => C, when you index C, you need to provide the id of A as the routing key but the id of B to set up the relation on the join field. In this case, NEST JoinRouting helper is unable to resolve to the id of A and will return the id of B.

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