How do you update objects in a document's array (nested updating) - mongodb

Question

Assume we have the following collection, which I have few questions about:
{
"_id" : ObjectId("4faaba123412d654fe83hg876"),
"user_id" : 123456,
"total" : 100,
"items" : [
{
"item_name" : "my_item_one",
"price" : 20
},
{
"item_name" : "my_item_two",
"price" : 50
},
{
"item_name" : "my_item_three",
"price" : 30
}
]
}
I want to increase the price for "item_name":"my_item_two" and if it doesn't exists, it should be appended to the "items" array.
How can I update two fields at the same time? For example, increase the price for "my_item_three" and at the same time increase the "total" (with the same value).
I prefer to do this on the MongoDB side, otherwise I have to load the document in client-side (Python) and construct the updated document and replace it with the existing one in MongoDB.
This is what I have tried and works fine if the object exists:
db.test_invoice.update({user_id : 123456 , "items.item_name":"my_item_one"} , {$inc: {"items.$.price": 10}})
However, if the key doesn't exist, it does nothing.
Also, it only updates the nested object. There is no way with this command to update the "total" field as well.

Answer 1

For question #1, let's break it into two parts. First, increment any document that has "items.item_name" equal to "my_item_two". For this you'll have to use the positional "$" operator. Something like:
db.bar.update( {user_id : 123456 , "items.item_name" : "my_item_two" } ,
{$inc : {"items.$.price" : 1} } ,
false ,
true);
Note that this will only increment the first matched subdocument in any array (so if you have another document in the array with "item_name" equal to "my_item_two", it won't get incremented). But this might be what you want.
The second part is trickier. We can push a new item to an array without a "my_item_two" as follows:
db.bar.update( {user_id : 123456, "items.item_name" : {$ne : "my_item_two" }} ,
{$addToSet : {"items" : {'item_name' : "my_item_two" , 'price' : 1 }} } ,
false ,
true);
For your question #2, the answer is easier. To increment the total and the price of item_three in any document that contains "my_item_three," you can use the $inc operator on multiple fields at the same time. Something like:
db.bar.update( {"items.item_name" : {$ne : "my_item_three" }} ,
{$inc : {total : 1 , "items.$.price" : 1}} ,
false ,
true);

Answer 2

There is no way to do this in single query. You have to search the document in first query:
If document exists:
db.bar.update( {user_id : 123456 , "items.item_name" : "my_item_two" } ,
{$inc : {"items.$.price" : 1} } ,
false ,
true);
Else
db.bar.update( {user_id : 123456 } ,
{$addToSet : {"items" : {'item_name' : "my_item_two" , 'price' : 1 }} } ,
false ,
true);
No need to add condition {$ne : "my_item_two" }.
Also in multithreaded enviourment you have to be careful that only one thread can execute the second (insert case, if document did not found) at a time, otherwise duplicate embed documents will be inserted.

Answer 3

We can use $set operator to update the nested array inside object filed update the value
db.getCollection('geolocations').update(
{
"_id" : ObjectId("5bd3013ac714ea4959f80115"),
"geolocation.country" : "United States of America"
},
{ $set:
{
"geolocation.$.country" : "USA"
}
},
false,
true
);

Answer 4

One way to ensure there are no duplicates of the "item_name" fields would be the do the same actions as given in the answer https://stackoverflow.com/a/10523963 for Question #1 but in reverse order!
Push into the document if "items.item_name":{"$ne":"my_name"} - the update filter MUST contain some uniquely indexed field! And upsert is false.
Increment the document if "items.item_name":"my_name".
The first update should be atomic, thus it doesn't do anything if the array already contains the element with the item_name "my_name".
By the time the second update happens, there must be an array element with the "item_name"="my_name"