[{"key":1},{"key":2},{"items":["abc","ab"]},{"_id":2,"items":["abc","bc"]},]

db.collection.aggregate([{"$project":{"original_items":"$items","a":{"$filter":{"input":"$items","as":"item","cond":{"$regexFind":{"input":"$$item","regex":"a","options":"i"}}}}}},{"$match":{"$and":[{$expr:{"$ne":["$a",null]}},{$expr:{"$eq":[{"$size":"$a"},{"$size":"$original_items"}]}}]}}])