mongodb - Get one array from two arrays in collection

Question

In my mongodb collection, I have sometimes two, sometimes one and sometimes null arrays on a document. Now I'd like to get one array over the whole collection with the values of these arrays.

The document looks like this:

{
    "title" : "myDocument",
    "listOne" : [ 
        "valueOne", 
        "valueTwo"
    ],
    "listTwo" : [ 
        "abc", 
        "qwer"
    ]
},
{
    "title" : "myDocumentTwo",
    "listTwo" : [  
        "321"
    ]
},
{
    "title" : "myDocumentAlpha",
    "listOne" : [ 
        "alpha", 
        "beta"
    ]
},
{
    "title" : "myDocumentbeta"
}

And I expect the following output:

"combinedList" : [
    "valueOne", 
    "valueTwo",
    "abc",
    "qwer",
    "321",
    "alpha",
    "beta"
]

It's like every possible value from these twos array out of every document in this collection.

"It's like every possible value from these twos array out of every document in this collection." Can you update your post to show that and what should be the final result? Right now you are only showing one document. — palaѕн
– palaѕн, Commented Jun 30, 2020 at 11:03
I updated it. The final result is an array with every possible value out of the other arrays. — marcos
– marcos, Commented Jun 30, 2020 at 11:10

palaѕн · Accepted Answer · 2020-06-30 11:22:53Z

2

You can do this using aggregate and $concatArrays

db.collection.aggregate([
  {
    $project: {
      combinedList: {
        $concatArrays: [{$ifNull: ["$listOne", []]}, {$ifNull: ["$listTwo", []]}]
      }
    }
  },
  { $unwind: "$combinedList" },
  { $group: { _id: null, combinedList: { $addToSet: "$combinedList"}}},
  { $project: { _id: 0, combinedList: 1 }}
])

answered Jun 30, 2020 at 11:22

palaѕн

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2 Comments

marcos Over a year ago

if the same value is in listOne and listTwo, is it possible to filter the combindedList that it's only displayed once?

palaѕн Over a year ago

That should be already working as we are using $group so you will always have distinct values in the combinedList

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