MongoDB setOnInsert and push if already existent
You could implement it without $setOnInsert operator.
db.test.update( { name : 'Peter' }, { $push : { "visits.en" : 'today' } }, { upsert : true })If Peter exists, element 'today' will be added to its visits.en array. Else, will be created a document for Peter, with visits object, that will be contain array en with 'today' element.
And I think, that error occured because of you using same property (visits) in two operations ($setOnInsert and $push).
You can still use $setOnInsert but when $setOnInsert and $push doesn't updates in the same fields as mentioned before.
N.b: We use $addToSet if you don't want a duplicated values in your Array
db.test.update( { name : 'Peter' }, { $setOnInsert: {name : 'Peter'}, $addToSet: {"visits.en": 'today'} // or $push }, {upsert: true})