N1QL (Couchbase) - True LEFT JOIN in couchbase for non-existing relationships - couchbase

Question

For simplicity's sake, lets say I have the following DB:
{
id: "a_id", // This is the key, not a field in the document
type: "a"
}
{
id: "b_id",
type: "b",
a: "a_id"
}
{
type: "c",
b: "b_id"
}
As you can see, there is a classic parent-child relationship ("a" HAS MANY "b", "b" HAS MANY "c")
Lets assume that there will not always be a type "c" document - for example, I have five "b" documents, but only one has a "c" document related to it.
I want to write a JOIN query that will give me all "b" that belong to "a", and if they exist - all "c" that belong to "b", but for "b" documents that don't have "c" I don't get any results - I would like to get all five "b" documents, as well as the one "c" document that is related.
My query looks like this:
SELECT *
FROM default AS a
JOIN default AS b ON KEY b.a FOR a
JOIN default AS c ON KEY c.b FOR b
WHERE a.type = "a"
AND b.type = "b"
AND c.type = "c";
What am I missing here?
Thanks!

Answer 1

WHERE clause post join filter if if c is MISSING c.type = “c” is false.
INSERT INTO default VALUES("a_id", { "type": "a" });
INSERT INTO default VALUES("b_id", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id1", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id1", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id2", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id3", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("b_id4", { "type": "b", "a":"a_id"});
INSERT INTO default VALUES("c_id", { "type": "c", "b":"b_id"});
CREATE INDEX ia ON default(a);
CREATE INDEX ib ON default(b);
SELECT *
FROM default AS d
LEFT JOIN default AS d1 ON KEY d1.a FOR d
LEFT JOIN default AS d2 ON KEY d2.b FOR d1
WHERE d.type = "a"
AND d1.type = "b"
AND (d2 IS MISSING OR d2.type = "c");
Also checkout https://dzone.com/articles/visually-explaining-n1ql-joins