How to make a correct aggregation request to an array to exclude an empty response-CodePudding

There is a database of this type: {_id: "...", shelf:{type:String}, books:{type:Array({ name :{type:String}})}} I need to extract all the shelves anyway, even if they don't have the book I'm looking for, my unsuccessful query looks like this:

let query = [ 
 {$match:{shelf: shelf}},
 {$unwind: "$books"}, 
 {$match: {$or:[ {'books.name ': {$exists: false}}, // If empty 
 {'books.name ': name}, // If there is a necessary book 
 {'books.name ': {$nin:name}} // If there is no necessary 
]}} ]

My desire to exclude an empty answer has led me to a dead end. there have been attempts to use $not, $ne, $nin. The result is one [ ] or undefined

CodePudding user response：

I believe you are trying to get the shelves irrespective of existence of books. Your query has $unwind which is stopping this you need to pass an additional parameter (preserveNullAndEmptyArrays) like this

let query = [{
    $match: {
      shelf: shelf
    }
  },
  {
    $unwind: {
      path: "$books",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $match: {
      $or: [{
          'books.name ': {
            $exists: false
          }
        }, // If empty 
        {
          'books.name ': name
        }, // If there is a necessary book 
        {
          'books.name ': {
            $nin: name
          }
        } // If there is no necessary 
      ]
    }
  }
]