[{"objectList":[{"toChange":0.0,"toMatch":"bar"}],"array":["first","second"]},{"objectList":[{"toChange":0.0,"toMatch":"bar"}],"array":["second","first"]}]

db.collection.update({array:{$in:["second"]},objectList:{$elemMatch:{toMatch:{$exists:true}}}},{$set:{"objectList.$[].toChange":10}},{multi:true})