mongo forEach unable to update array in document - mongodb

Question

I'm trying write a mongo script to run in RoboMongo that will loop through all documents in a collection. Each document contains an array myArray. The documents look like this:
{
"name": "myApp",
"myArray": [
{ "env": "dev", "dbHost": "db2dev.local" },
{ "env": "prod", "dbHost": "db1prod.local" }
]
I want to copy the dbHost field that is defined in dev to prod. So the above result would be:
{
"name": "myApp",
"myArray": [
{ "env": "dev", "dbHost": "db2dev.local" },
{ "env": "prod", "dbHost": "db2dev.local" }
]
When I try to access the field myArray[0] I get a syntax error that says:
TypeError: myDoc.myArray[0] is undefined
The function is something like this:
db.myCollection.find().forEach( function(myDoc) {
var devIdx = 0;
var prodIdx = 1;
if (myDoc.myArray[0].env !== 'dev')}
devIdx = 1;
prodIdx = 0;
}
myDoc.myArray[prodIdx].dbHost = myDoc.myArray[devIdx].dbHost;
print(myDoc);
});
I've examined the collection (it is very small) and each document has a myArray field as it should with exactly two values (one for dev and one for prod) in the array.
What am I doing wrong? What is the correct syntax to use inside a mongo script? Is updating arrays in a document not supported?
Searching for solution
I've searched and found forEach examples but most are trivial and none include an array being accessed or changed.
The mongo docs are also very simplistic: https://docs.mongodb.com/v3.6/reference/method/cursor.forEach/

Answer 1

Mongo javascript does not allow you to access arrays directly like you are trying to do (unless you are in a for loop). So a solution is shown below:
db.myCollection.find({}).forEach( function(myDoc) {
var foundDevEntry = null;
var updatedProdEntry = false;
// First time loop to get a copy of the dev entry
for (var idx in myDoc.myArray) {
if (myDoc.myArray[idx].env === 'dev') {
foundDevEntry = myDoc.myArray[idx];
}
}
// 2nd time loop to update the value
for (var idx in myDoc.myArray) {
if (myDoc.myArray[idx].env === 'prod') {
myDoc.myArray[idx].dbHost = foundDevEntry.dbHost;
}
}
// Now update the database with this change
db.myCollection.update({_id: myDoc._id}, {$set: {"myArray": myDoc.myArray}});
print(myDoc); // So results are also returned when query is run
});
I've stripped out error checking to focus on the change required. What (to me) is odd is that the syntax myDoc.myArray[idx] is actually valid but only inside a loop!
The following references helped me come to a solution:
Update in forEach on mongodb shell
https://www.mysoftkey.com/mongodb/how-to-use-foreach-loop-in-mongodb-to-manipulate-document/
I should add that some solutions I read said that to update an array you had to re-build the array (https://stackoverflow.com/a/22657219/3281336). I did not do that in my solution and it did work but wanted to share it.