MongoDB find if all array elements are in the other bigger array - mongodb

Question

I have an array of id's of LEGO parts in a LEGO building.
// building collection
{
"name": "Gingerbird House",
"buildingTime": 45,
"rating": 4.5,
"elements": [
{
"_id": 23,
"requiredElementAmt": 14
},
{
"_id": 13,
"requiredElementAmt": 42
}
]
}
and then
//elements collection
{
"_id": 23,
"name": "blue 6 dots brick",
"availableAmt":20
}
{
"_id": 13,
"name": "red 8 dots brick",
"availableAmt":50
}
{"_id":254,
"name": "green 4 dots brick",
"availableAmt":12
}
How can I find it's possible to build a building? I.e. database will return the building only if the "elements" array in a building document consists of those elements that I have in a warehouse(elements collection) require less(or equal) amount of certain element.
In SQL(which from I came recently) I would write something likeSELECT * FROM building WHERE id NOT IN(SELECT fk_building FROM building_elemnt_amt WHERE fk_element NOT IN (1, 3))
Thank you in advance!

Answer 1

I wont pretend I get how it works in SQL without any comparison, but in mongodb you can do something like that:
db.buildings.find({/* building filter, if any */}).map(function(b){
var ok = true;
b.elements.forEach(function(e){
ok = ok && 1 == db.elements.find({_id:e._id, availableAmt:{$gt:e.requiredElementAmt}}).count();
})
return ok ? b : false;
}).filter(function(b){return b});
or
db.buildings.find({/* building filter, if any */}).map( function(b){
var condition = [];
b.elements.forEach(function(e){
condition.push({_id:e._id, availableAmt:{$gt:e.requiredElementAmt}});
})
return db.elements.find({$or:condition}).count() == b.elements.length ? b : false;
}).filter(function(b){return b});
The last one should be a bit quicker, but I did not test. If performance is a key, it must be better to mapReduce it to run subqueries in parallel.
Note: The examples above work with assumption that buildings.elements have no elements with the same id. Otherwise the array of elements needs to be pre-processed before b.elements.forEach to calculate total requiredElementAmt for non-unique ids.
EDIT: How it works:
Select all/some documents from buildings collection with find:
db.buildings.find({/* building filter, if any */})
returns a cursor, which we iterate with map applying the function to each document:
map(function(b){...})
The function itself iterates over elements array for each buildings document b:
b.elements.forEach(function(e){...})
and find number of documents in elements collection for each element e
db.elements.find({_id:e._id, availableAmt:{$gte:e.requiredElementAmt}}).count();
which match a condition:
elements._id == e._id
and
elements.availableAmt >= e.requiredElementAmt
until first request that return 0.
Since elements._id is unique, this subquery returns either 0 or 1.
First 0 in expression ok = ok && 1 == 0 turns ok to false, so rest of the elements array will be iterated without touching the db.
The function returns either current buildings document, or false:
return ok ? b : false
So result of the map function is an array, containing full buildings documents which can be built, or false for ones that lacks at least 1 resource.
Then we filter this array to get rid of false elements, since they hold no useful information:
filter(function(b){return b})
It returns a new array with all elements for which function(b){return b} doesn't return false, i.e. only full buildings documents.