Spring Data MongoDB index declaration for field of field - mongodb

Question

I have the following Spring Data MongoDB document:
#Document(collection = "messages")
#CompoundIndexes({ #CompoundIndex(name = "chatId_messageId", def = "{'chatId': 1, 'messageId': 1}") })
public class Message implements Serializable {
private static final long serialVersionUID = -1590112217771926691L;
#Id
private String id;
private Long chatId;
private Integer messageId;
private Post post;
}
the Post model(which is not Spring Data MongoDB document) looks like:
public class Post implements Serializable {
private static final long serialVersionUID = -886664249197573502L;
private String id;
}
I'd like to add index to the Message.post.id field.
How it can be done with Spring Data MongoDB and Message.post field declaration in Message document ?

Answer 1

If you want to add Message.post.id to an already compound index, then do it like
#Document(collection = "messages")
#CompoundIndexes({ #CompoundIndex(name = "chatId_messageId", def = "{'chatId': 1, 'messageId': 1, 'Message.post.id' : 1}") })
public class Message implements Serializable {
private static final long serialVersionUID = -1590112217771926691L;
#Id
private String id;
private Long chatId;
private Integer messageId;
private Post post;
}
Compound indexes are indexes that have more than one indexed field, so ideally, the most restrictive field should be to the left of the B-tree. If you want to index by sex and birth, for instance, the index should begin by birth, as it is much more restrictive than sex.
Or if you want to treat it as a saperate index, then create index using #Indexed like
public class Post implements Serializable {
private static final long serialVersionUID = -886664249197573502L;
#Indexed
private String id;
}
Updated
For more info regarding how queries with sub fields of compound index works, check Documentation