MongoDB：检查该集合中所有文档中的字段是否相同

Question

How could I check whether a specific field's value if same in all the document in that collection.

I have a collection called Game where I store the game responses of the users and in the game, the logic should check whether the responses are all equal or not, so how could I do it?

Answer 1

You need to do the opposite

find({field: {$ne: expectedValue}})

If you get a response for this query, then there are documents which is not having the desired value.

Answer 2

You can get the count of unique values of that field. If more than 1, they are not the same.

You can also group by the field value. If more than 1 record, they are not the same.

const rows = await myCollection.aggregate([
 {
  $group: {
    _id: '$theField'
  }
 }
]).toArray()

if (rows.length === 1){
  // same field value
}

Or do the counting in the pipeline to save bandwidth.

const rows = await myCollection.aggregate([
 {
  $group: {
    _id: '$theField'
  }
 },
 { $count: 'count' }
]).toArray()

if (rows.length && rows[0].count === 1) {
  // same value
}