Search in an array of an embedded object in MongoDB - mongodb

Question

Given this structure for a school object:
{
"grade_spans" :
{
"0": {
"grade_span_key" : "K_5",
"name": "Elementary School"
},
"1": {
"grade_span_key" : "6_8",
"name": "Junior High-School"
}
}
}
How do I find a school for a given grade_span_key?
db.schools.find({ "grade_span_key": "K_5" })
returns empty.
Update: Sorry, I copied the structure incorrectly. It's actually an Embedded Object not a collection.
Update #2: There was a doctrine2 annotation I was using incorrectly: #MongoDB\EmbedMany(strategy="set"). I change the strategy to pushAll (which is the default)

Answer 1

If this field is just embedded into the main document #sergios answer will work just fine and it is not clear why his query wouldn't work as you don't provide an example of the document structure only of the embedded structure.
Also as #JohnnyHK says, rebuild that object as an array since dynamic keys in this case would be harder.
If you are looking to pick out matching rows from the embedded document and not the full document. This is a little harder but is possible:
db.schools.aggregate({
{$unwind: "$grade_spans"},
{$match: {"grade_spans.grade_span_key": "K_5"}},
{$group: {_id: "$_id", grade_spans: {$push: "$grade_spans"}}}
})
Something like the above should return a document of the structure:
{
_id: {},
grade_spans:[{
"grade_span_key" : "K_5",
"name" : "Elementary School"
}]
}

Answer 2

You should use full path to the property, in dotted notation.
> db.schools.find({"grade_spans.grade_span_key": "K_5"})
{
"_id" : ObjectId("50801cc5ab582e310adc0e41"),
"grade_spans" : [
{
"grade_span_key" : "K_5",
"name" : "Elementary School"
},
{
"grade_span_key" : "6_8",
"name" : "Junior High-School"
}
]
}

Answer 3

Given this structure :
{
"grade_spans" : {
"0": { "grade_span_key" : "K_5",
"name": "Elementary School" },
"1": { "grade_span_key" : "6_8",
"name": "Junior High-School" }
}
}
You can try with map/reduce function :
var mapFunction = function() {
for (var i in this.grade_spans) {
// Add the name of the school in a list
var schools = [];
schools[0] = this.grade_spans[i].name;
// Create out object : { schools : ["Elementary School"] } or { schools : ["Junior High-School"] }
var out = {};
out.schools = schools;
// Create key (K_5 or 6_8)
var key = this.grade_spans[i].grade_span_key;
emit(key, out);
}
};
var reduceFunction = function(key, values) {
var schools = [];
for (var i = 0; i < values.length; i++) {
schools.push.apply(schools, values[i].schools);
}
return {schools:schools};
}
db.schools.mapReduce(
mapFunction,
reduceFunction,
{ out: "map_reduce_grade_spans", sort: {_id:1} }
)
And then :
db.map_reduce_grade_spans.find({_id:"K_5"});