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LoginGeneral Tech Bugs & Fixes 3 years ago
Posted on 16 Aug 2022, this text provides information on Bugs & Fixes related to General Tech. Please note that while accuracy is prioritized, the data presented might not be entirely correct or up-to-date. This information is offered for general knowledge and informational purposes only, and should not be considered as a substitute for professional advice.
You just need to run query
db.test.find(
{"shapes.color": "red"},
{shapes: {$elemMatch: {color: "red"}}});output of this query is
{
"_id" : ObjectId("562e7c594c12942f08fe4192"),
"shapes" : [
{"shape" : "circle", "color" : "red"}
]
}as you expected it'll gives the exact field from array that matches color:'red'.
Better you can query in matching array element using $slice is it helpful to returning the significant object in an array.
db.test.find({"shapes.color" : "blue"}, {"shapes.$" : 1})$slice is helpful when you know the index of the element, but sometimes you want whichever array element matched your criteria. You can return the matching element with the $ operator.
The syntax for find in mongodb is
db.<collection name>.find(query, projection);and the second query that you have written, that is
db.test.find(
{shapes: {"$elemMatch": {color: "red"}}},
{"shapes.color":1})in this you have used the $elemMatch operator in query part, whereas if you use this operator in the projection part then you will get the desired result. You can write down your query as
db.users.find(
{"shapes.color":"red"},
{_id:0, shapes: {$elemMatch : {color: "red"}}})This will give you the desired result.
Here I just want to add some more complex usage.
// Document
{
"_id" : 1
"shapes" : [
{"shape" : "square", "color" : "red"},
{"shape" : "circle", "color" : "green"}
]
}
{
"_id" : 2
"shapes" : [
{"shape" : "square", "color" : "red"},
{"shape" : "circle", "color" : "green"}
]
}
// The Query
db.contents.find({
"_id" : ObjectId(1),
"shapes.color":"red"
},{
"_id": 0,
"shapes" :{
"$elemMatch":{
"color" : "red"
}
}
})
//And the Result
{"shapes":[
{
"shape" : "square",
"color" : "red"
}
]}Another interesing way is to use $redact, which is one of the new aggregation features of MongoDB 2.6. If you are using 2.6, you don't need an $unwind which might cause you performance problems if you have large arrays.
db.test.aggregate([
{ $match: {
shapes: { $elemMatch: {color: "red"} }
}},
{ $redact : {
$cond: {
if: { $or : [{ $eq: ["$color","red"] }, { $not : "$color" }]},
then: "$$DESCEND",
else: "$$PRUNE"
}
}}]);$redact "restricts the contents of the documents based on information stored in the documents themselves". So it will run only inside of the document. It basically scans your document top to the bottom, and checks if it matches with your if condition which is in $cond, if there is match it will either keep the content($$DESCEND) or remove($$PRUNE).
In the example above, first $match returns the whole shapes array, and $redact strips it down to the expected result.
Note that {$not:"$color"} is necessary, because it will scan the top document as well, and if $redact does not find a color field on the top level this will return false that might strip the whole document which we don't want.
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manpreet![Tuteehub forum best answer]()
Best Answer
3 years ago
Retrieve only the queried element in an object array in MongoDB collection
Suppose you have the following documents in my collection:
{ "_id":ObjectId("562e7c594c12942f08fe4192"), "shapes":[ { "shape":"square", "color":"blue" }, { "shape":"circle", "color":"red" } ] }, { "_id":ObjectId("562e7c594c12942f08fe4193"), "shapes":[ { "shape":"square", "color":"black" }, { "shape":"circle", "color":"green" } ] }Do query:
db.test.find({"shapes.color": "red"}, {"shapes.color": 1})Or
db.test.find({shapes: {"$elemMatch": {color: "red"}}}, {"shapes.color": 1})Returns matched document (Document 1), but always with ALL array items in shapes:
{ "shapes": [ {"shape": "square", "color": "blue"}, {"shape": "circle", "color": "red"} ] }However, I'd like to get the document (Document 1) only with the array that contains color=red:
{ "shapes": [ {"shape": "circle", "color": "red"} ] }How can I do this?