MongoDB update array if condition matches - mongodb

Question

I'm looking for a MongoDB aggregation pipeline which updates an array with a conditional statement.
My data looks like the following:
{
"_id": 1,
"locations": [
{
"controllerID": 1,
"timestamp": 1234
},
{
"controllerID": 2,
"timestamp": 2342
}
]
},...
Potential new entry:
{
"controllerID": 2,
"timestamp": //will be set automatically
}
At first I want to match the _id (not a problem) and then push the new entry to the locations array if the element with the newest/latest timestamp has a different controllerID.
When pushing a new location object the timestamp will be set automatically.
Example 1
Input:
{
"controllerID": 2,
}
Expected Result:
{
"_id": 1,
"locations": [
{
"controllerID": 1,
"timestamp": 1234
},
{
"controllerID": 2,
"timestamp": 2342
}//noting is added because the newset entry in the array has the same controllerID
]
},
Example 2
Input:
{
"controllerID": 1,
}
Expected Result:
{
"_id": 1,
"locations": [
{
"controllerID": 1,
"timestamp": 1234
},
{
"controllerID": 2,
"timestamp": 2342
},
{//added because the controllerID is different to te last element
"controllerID": 1,
"timestamp": 4356
}
]
},
Thanks in advance!

Answer 1

Here's a solution.
var candidate = 2;
rc=db.foo.update({}, // add matching criteria here; for now, match ALL
[
[
// We cannot say "if condition then set fld = X else do nothing".
// We must say "set fld to something based on condition."
// The common pattern becomes:
// "Set fld to (if condition then X else fld)"
// in other words, set the fld to *itself*
//
// Note the use of the dot operator on the $locations field.
// Also, not sure about what sort of new timestamp is desired so let's
// just throw in an ISODate() for now.
{$set: {'locations': {$cond: [
{$ne:[candidate, {$last:'$locations.controllerID'}]}, // IF not same as candidate...
{$concatArrays: ['$locations',
// $concatArrays wants arrays, not objects, so we must wrap our new
// object with [] to make an array of 1:
[ {controllerId:candidate,timestamp:new ISODate() } ]
]}, // THEN concat a new entry to end of existing locations
'$locations' // ELSE just set back to existing locations
]}
}}
],
{multi:true}
);
The engine is "smart enough" to realize that setting a field to itself will not trigger a modification so the approach is performant and will not rewrite the entire set of matched objects; this can be seen in the output of the update() call, e.g.:
printjson(rc);
{ "nMatched" : 1002, "nUpserted" : 0, "nModified" : 1 }