mongodb - How can I merge two field arrays in many documents into a single set? -

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i have mongodb collection data looks this:

{ "_id" : "1", "array1" : [ "1", "2" ] }, { "_id" : "2", "array2" : [ "1", "3" ] }, { "_id" : "3", "array1" : [ ] }, { "_id" : "4", "array2" : [ ] }, { "_id" : "5" }, { "_id" : "6", "array1" : [ "3", "4" ], "array2" : [ "5" ] }

i find query returns unique array values in single array so:

{"_id":"theid", "result":["1", "2", "3", "4", "5"]}

the id isn't important. note either array1, array2, both or neither can present in document , can empty. have tried many aggregations , cascading query commands , can't come desired response.

to need use .aggregate() method provides access aggregation pipeline.

the first stage in pipeline uses $match operator filter out documents both array1 , array2 not presents using $exists operator , dot notation. operator reduce number of documents processed in down in pipeline.

the next stage $project use $setunion return array containing elements appear in of array; filters out duplicates elements in result. not use of $ifnull operator returns value of first expression or empty array depending on whether first expression evaluates null (here expression "array1" , "array2"). there, need de-normalize "arrays" field using $unwind operator.

in last stage of pipeline $group , use $addtoset accumulator operator returns array of unique value. 

db.getcollection('collection').aggregate([     { "$match": {          "$or": [              { "array1.0": { "$exists": true } },              { "array2.0": { "$exists": true } }         ]     }},      { "$project": {          "arrays": {              "$setunion": [                  { "$ifnull": [ "$array1", [] ] },                 { "$ifnull": [ "$array2", [] ] }              ]          }     }},      { "$unwind": "$arrays" },      { "$group": {          "_id": null,          "arrays": { "$addtoset": "$arrays" }      }}  ] )

which yields:

{ "_id" : null, "arrays" : [ "5", "3", "1", "4", "2" ] }

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