How can query in MongoDB that count number of 2 equal field? - mongodb

Question

I have a collection in MongoDB for my survey results(name=surveyresults). I want to have a query that gives me the number of correct answers based on category, for example, category "Bee" number of correct answers 10.
I tried different ways but these are not results that I want.
I have searched and found this post Group count with MongoDB using aggregation framework useful but not working for me.
This is part of my data in my surveyResults collection :
[{"_id":"0eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJfaWQiOjE5LCJpYXQiOjE1MjQwMDgzOTl9.2YvhnXtCD7-fm4B14k10m6NF7xuv7moCTbekVekkbvY","category":"Wasp","photo":"A_wasp_565","description":"","answer":"Bee","__v":0},{"_id":"1eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJfaWQiOjE5LCJpYXQiOjE1MjQwMDgzOTl9.2YvhnXtCD7-fm4B14k10m6NF7xuv7moCTbekVekkbvY","category":"Wasp","photo":"A_Pompilid_wasp_007","description":"","answer":"Wasp","__v":0},{"_id":"2eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJfaWQiOjE5LCJpYXQiOjE1MjQwMDgzOTl9.2YvhnXtCD7-fm4B14k10m6NF7xuv7moCTbekVekkbvY","category":"Wasp","photo":"wasp_248","description":"","answer":"Wasp","__v":0},{"_id":"3eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJfaWQiOjE5LCJpYXQiOjE1MjQwMDgzOTl9.2YvhnXtCD7-fm4B14k10m6NF7xuv7moCTbekVekkbvY","category":"Fly","photo":"A_butterfly_291","description":"kjlkjlkjlk","answer":"Moth/Butterfly","__v":0},
I want result like this :
[{"category":"Fly","count":3, "correct":1},{"category":"Wasp","count":3, "correct":1},{"category":"Moth/Butterfly","count":4, "correct":2},{"category":"Bee","count":3, "correct":1}]
Now I have these two queries but not giving me correct results :
1.
SurveyResults.aggregate([
{ $group: {
_id: { answer: '$answer', category: '$category' }
}},
{ $group: {
_id: '$_id.answer',
answer_correct: { $sum: 1 }
}},
{ $project: {
_id: 0,
answer: '$_id',
answer_correct: 1
}}
]).exec(callback);
2.
SurveyResults.aggregate([
{
$group:{
_id:"$answer",
count: { $sum : {$cond : { if: { $eq: ["answer", "$category"]}, then: 1, else: 0} }}
}
}]).exec(callback);
Also, I can have the number of answers based on the category by this query:
SurveyResults.aggregate([
{
$group:{
_id:"$answer",
count: { $sum : 1 }
}
}]).exec(callback);
Results:
[{"_id":"Don't know","count":2},{"_id":"Fly","count":3},{"_id":"Wasp","count":3},{"_id":"Moth/Butterfly","count":4},{"_id":"Bee","count":3}]

Answer 1

Here's what you want:
SurveyResults.aggregate([
$group: {
_id: "$category",
"count": { $sum: 1 }, // simply count all questions per category
"correct": {
$sum: { // and sum up the correct ones in a field called "correct"
$cond: [ // ...where "correct ones" means
{ $eq: [ "$category", "$answer" ] }, // that "category" needs to match "answer"
1,
0
]
}
}
}
}, {
$project: { // this is just to effectively rename the "_id" field into "category" - may or may not be needed
_id: 0,
"category": "$_id",
"count": "$count",
"correct": "$correct"
}
}]).exec(callback);