Remove multiple documents from array in MongoDB - mongodb

Question

My document contains an array like:
{
"differentialDiagnosis" : "IART/Flutter",
"explanation" : "The rhythm.",
"fileName" : "A115a JPEG.jpg",
"history" : "1 year old with fussiness",
"interpretationList" : [
{
"interpretations" : [
ObjectId("54efe7c8d6d5ca3d5c580a22"),
ObjectId("54efe80bd6d5ca3d5c580a26")
]
},
{
"interpretations" : [
ObjectId("54efe80bd6d5ca3d5c580a26"),
ObjectId("54efe82ad6d5ca3d5c580a28")
]
}
],
}
and I want to remove all occurrences of ObjectId("54efe80bd6d5ca3d5c580a26"),
but I write a query:
db.ekgs.update({'interpretationList.interpretations':ObjectId("54c09fb3581c4c8c218d1a40")}, {$pull:{ 'interpretationList.$.interpretations':{ ObjectId("54c09fb3581c4c8c218d1a40")}})
This removes only first occurrence of ObjectId("54efe80bd6d5ca3d5c580a26").

Answer 1

The reason your query is only removing the first occurrence is because, as explained in this page in the documentation, "the positional $ operator acts as a placeholder for the first element that matches the query document".
The problem is that it is really tricky to deal with these types of updates with schema having embedded arrays in embedded objects in embedded arrays. In order to get around this problem, if you are able to flatten the schema, then your update becomes much easier. So if instead, your document looked like this:
{
"differentialDiagnosis" : "IART/Flutter",
"explanation" : "The rhythm.",
"fileName" : "A115a JPEG.jpg",
"history" : "1 year old with fussiness",
"interpretations" : [
ObjectId("54efe7c8d6d5ca3d5c580a22"),
ObjectId("54efe80bd6d5ca3d5c580a26"),
ObjectId("54efe82ad6d5ca3d5c580a28")
]
}
Then your query would be as simple as the one below. (Remember to add { "multi": true } as an option if you want to update multiple documents).
db.ekgs.update(
{ "interpretations": ObjectId("54efe80bd6d5ca3d5c580a26")},
{ "$pull": { "interpretations": ObjectId("54efe80bd6d5ca3d5c580a26") }}
);
But I understand that you might not be able to change the schema. In that case, you can try a solution that requires a small script. In the mongo shell, you can use the following bit of JavaScript to do the operation.
// Get cursor with documents requiring updating.
var oid = ObjectId("54efe80bd6d5ca3d5c580a26");
var c = db.ekgs.find({ "interpretationList.interpretations": oid });
// Iterate through cursor, removing oid from each subdocument in interpretationList.
while (c.hasNext()) {
var isModified = false;
var doc = c.next();
var il = doc.interpretationList;
for (var i in il) {
var j = il[i].interpretations.length;
while (j--) {
// If oid to remove is present, remove it from array
// and set flag that the document has been modified.
if (il[i].interpretations[j].str === oid.str) {
il[i].interpretations.splice(j, 1);
isModified = true;
}
}
}
// If modified, update interpretationList for document.
if (isModified) {
db.ekgs.update({ "_id": doc._id }, { "$set": { "interpretationList": il }});
}
}
UPDATE: Example of how it might work using the Node.js driver.
// Get cursor with documents requiring updating.
var oid = new ObjectID("54efe80bd6d5ca3d5c580a26");
var ekgs = db.collection("ekgs");
ekgs.find({ "interpretationList.interpretations": oid },
function(err, c) {
if(err) throw err;
// Iterate through cursor, removing oid from each subdocument in interpretationList.
c.each(function(err, doc) {
if (err) throw err;
// If doc is null then the cursor is exhausted/empty and closed.
if (doc != null) {
var isModified = false;
var il = doc.interpretationList;
for (var i in il) {
var j = il[i].interpretations.length;
while (j--) {
// If oid to remove is present, remove it from array
// and set flag that the document has been modified.
if (il[i].interpretations[j].equals(oid)) {
il[i].interpretations.splice(j, 1);
isModified = true;
}
}
}
// If modified, update interpretationList for document.
if (isModified) {
ekgs.update({ "_id": doc._id },
{ "$set": { "interpretationList": il }},
function(err, res) {
if (err) throw err;
// Callback.
console.log(res);
});
}
}
});
});