ArangoDB AQL Query - merge children into parent - arangodb

Question

I have a simple parent-child relationship in ArangoDB. Each parent document can have zero to many children. Let's say the parent document has attributes pa1, and the child docs have one attribute ca1, and a reference back to the parent _id of "_id_parent". How do I write an AQL query to return a result set like:
[{
"_key": "111",
"_id": "parent/111",
"pa1": "aaa",
"children": [{
"_key": "21",
"_id": "child/21",
"_id_parent": "parent/111",
"ca1": "www"
},
{
"_key": "22",
"_id": "child/22",
"_id_parent": "parent/111",
"ca1": "xxx"
}
]
},
{
"_key": "222",
"_id": "parent/222",
"pa1": "ddd",
"children": [{
"_key": "31",
"_id": "child/31",
"_id_parent": "parent/222",
"ca1": "yyy"
},
{
"_key": "32",
"_id": "child/32",
"_id_parent": "parent/222",
"ca1": "zzz"
}
]
}
]
In other words, how do I "flatten" this:
FOR p IN Parent
FILTER p.pa1 == #parm1
LET children = (
(FOR c IN Child
FILTER c._id_parent == p._id
RETURN c)
)
RETURN {p, children}

Answer 1

All what's left to do is to actually merge the children with the parent document:
RETURN MERGE(p, {children})
That is the short form of RETURN MERGE(p, {children: children}).
p is this:
{
"_id": "Parent/111",
"_key": "111",
"_rev": "_WLsUlK2--_",
"pa1": "aaa"
}
{children} or the more verbose {children: children} creates an object with a single attribute, with the name children and the value of the children variable:
{
"children": [
{
"_key": "21",
"_id": "Child/21",
"_rev": "_WLsW4Su--_",
"_id_parent": "Parent/111",
"ca1": "www"
},
{
"_key": "22",
"_id": "Child/22",
"_rev": "_WLsW8Zu--_",
"_id_parent": "Parent/111",
"ca1": "xxx"
}
]
}
MERGE() combines both objects. Since there is no overlap in attribute keys ("_id", "_key", "_rev", "pa1" vs. "children"), no top level attributes of p are replaced by children.
BTW: You can spare one pair of parentheses around the subquery expression
LET var = ( ( <subquery> ) ) - it is sufficient to do LET var = ( <subquery> ).