Delete collection in mongodb - database

Question

I have the following json file in mongodb:
{
"_id" : ObjectId("59de156faf75d539b47e8db3"),
"user" : "user1",
"item" : {
"32a1fsd32asfd65asdf65" : {
...
},
"32a1fsd32asfd555" : {
}, ...
}
}
I want to perform a query and delete one of the two items. As a matter of fact, my database contains several users. Therefore, in order to retrieve the specific one from the mongodb i am performing the following:
How can I retrieve also a specific item and delete all its fields (for example 32a1fsd32asfd65asdf65)?

Answer 1

Based on the example document provided in your document it looks like you want to remove an attribute of the subdocument item.
You can use the $unset update operator:
db.getCollection('colName').update(
// find a specific document
{user: 'user1'},
// unset the attribute named "sfd65asdf65"
{$unset: {'item.sfd65asdf65': 1}}
)
Given the document provided in your question, the above command will cause that document to be updated to:
{
"_id" : ObjectId("59de156faf75d539b47e8db3"),
"user" : "user1",
"item" : {
"sd32asfd555" : {
...
}
}
}
If you want to remove the item attribute entirely then you would run:
db.getCollection('colName').update(
// find a specific document
{user: 'user1'},
// unset the attribute named "32a1fsd32asfd65asdf65"
{$unset: {'item': 1}}
)
And if you want to empty the item attribute (i.e. remove all of its attributes but retain the item attribute) then you would run this command:
db.getCollection('colName').update(
// find a specific document
{user: 'user1'},
// overwrite the "item" attribute with an empty sub document
{$set: {'item': {}}}
)
You can find more examples in the docs.

Answer 2

Say we have some users:
> db.test.insert( { _id: 1, user: "bob", field1: 1, field2 :{ field3 : 2 } } )
WriteResult({ "nInserted" : 1 })
> db.test.insert( { _id: 2, user: "fred", field1: 1, field2 :{ field3 : 2 } } )
WriteResult({ "nInserted" : 1 })
We can then use replaceOne to find the user and also then just replace the whole document, thus removing all the fields from that document:
> db.test.replaceOne( { user: "bob"}, { user: "bob" } )
{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }
Then our new documents will look like:
> db.test.find()
{ "_id" : 1, "user" : "bob" }
{ "_id" : 2, "user" : "fred", "field1" : 1, "field2" : { "field3" : 2 } }