Query collection containing BsonArray - arrays

Question

Sorry, first time trying mongo.
Given the following data ...
db.masterList.findOne()
{
"_id" : ObjectId("59d128805b19310ac8ab3fc2"),
"MasterDefinition" : {
"Location" : [
"Whole House",
"Master Bedroom",
"Hallway 2"
],
"DeviceType" : [
"Receptacle",
"GFI",
"LED dimmer"
],
"Style" : [
"Decora",
"Standard"
],
"Color" : [
"White",
"Light Almond"
]
}
}
How do I retrieve the contents of the Color array? I expect something like
["White","Light Almond"]
How do I list the 4 arrays directly subordinate to MasterDefintion? I expect to see
["Location","DeviceType","Style","Color"]
Thanks

Answer 1

For the first part, you can simply do a
collection.aggregate({
$project: {
"_id": 0, // exclude the "_id" field from the result
"result": "$MasterDefinition.Color"
}
})
The second part requires a little magic (documentation can be found here: aggregation framework, $project, $objectToArray):
collection.aggregate({
$project: {
"temp": {
$objectToArray: "$MasterDefinition" // transform the "MasterDefinition" subdocument into an array
}
}
}, {
$project:{
"_id": 0, // do not include the "_id" field in the result - this is an optional step
"result": "$temp.k" // only get the keys (as in "k" fiels) from the array
}
})

Answer 2

How do I retrieve the contents of the Color array? I expect something like ["White","Light Almond"]
// the first argument here is a filter, the second argument is a projection
// since you specified no filter I have only included a projection
// this projection tells MongoDB to return the Color subdocument
// from within the MasterDefinition sub document
db.getCollection('masterList').find({}, {'MasterDefinition.Color': 1})
The above command will return:
{
"_id" : ObjectId("59d128805b19310ac8ab3fc2"),
"MasterDefinition" : {
"Color" : [
"White",
"Light Almond"
]
}
}
How do I list the 4 arrays directly subordinate to MasterDefintion? I expect to see ["Location","DeviceType","Style","Color"]
This is a bit trickier because "Location","DeviceType","Style","Color" are not elements in an array instead they are the names of attributes in the MasterDefinition subdocument. You can use the $objectToArray aggregation operator to turn these attribute names into an array but the resulting document doesn't looks exactly like what you hoped for. Here's an example ...
db.getCollection('masterList').aggregate([
// creates an array named "categories" from the attributes of the MasterDefinition sub document
{ $project: { categories: { $objectToArray: "$MasterDefinition" } } },
// projects on the keys of the "categories" array
{$project: {'categories.k': 1}}
])
... which produces this output:
{
"_id" : ObjectId("59d128805b19310ac8ab3fc2"),
"categories" : [
{
"k" : "Location"
},
{
"k" : "DeviceType"
},
{
"k" : "Style"
},
{
"k" : "Color"
}
]
}