Using Mongo shell, I am trying to add a field which has the same value as an existing field for all documents in a collection. Assuming we have two documents :
{
'foo': 'str1'
}
and
{
'foo': 'str2'
}
I would like to insert a new field 'foo_new' which has the respective value of 'foo' as its value, so that the documents become
{
'foo': 'str1'
'foo_new': 'str1'
}
and
{
'foo': 'str2'
'foo_new': 'str2'
}
The command I use to update the collection ('coll' say) in Mongo shell is
db.coll.update({}, {$set: {'foo_new': '$foo'}}, {multi: true})
The result of running this command are the two updated documents
{
'foo': 'str1'
'foo_new': '$foo'
}
and
{
'foo': 'str2'
'foo_new': '$foo'
}
i.e. '$foo' is being interpreted as a literal for some reason.