How do I update MongoDB document fields only if they don't exist?

Question

I have collection foo with documents like:

{site_id: 'xxx', title: {ru: 'a', en: 'b'}, content: {ru: 'a', en: 'b'}} {site_id: 'xxx', title: {ru: 'c', de: 'd'}, content: {ru: 'c', de: 'd'}} 

I need to update multiple fields which are can exists or not:

db.foo.update(     { site_id: 'xxx'},     { $set: {'title.de': '', 'content.de': ''}},     {multi: true} ) 

But I need something like $set which will not overwrite value if it exists.

Answer 1

You can add a query to your update statement:

db.foo.update({'title.de': {$exists : false}}, {$set: {'title.de': ''}}) 

Update

For your modified question my solution looks like this - would that work for you? (If not, why?)

db.foo.update({site_id: 'xxx', 'title.de': {$exists : false}}, {$set: {'title.de': ''}, {multi: true}) db.foo.update({site_id: 'xxx', 'content.de': {$exists : false}}, {$set: {'content.de': ''}}, {multi: true})