Get keys of fields from Embedded Array of Documents Mongodb Java

Here is a simplified schema of my MongoDB doc:


"_id": 0,
"config": [
"property1": "a",
"property2": "b",
"property3": "c",
"property4": "d"
,

"property1": "a",
"property2": "bb",
"property3": "cc",
"property4": "d",
"ispropert5": true
,

"property1": "a",
"property2": "b",
"property3": "c",
"property4": "ddd",
"ispropert5": false,
"ispropert6": false

],
"entity": "123asdf",
"url": "",
"createdDate": 1



As an output I need to get a list of unique keys of nested documents:
property1, property2, property3, property4, ispropert5, ispropert6

I am trying this in my class but of course failing to cast ArrayList to Document:

Document dbo = col.find().first();
Set<String> keys = dbo.keySet();
Iterator iterator = keys.iterator();

while(iterator.hasNext())
String key = iterator.next().toString();
if(dbo.get(key) instanceof ArrayList)
Document dboNested = (Document) dbo.get(key); //java.lang.ClassCastException: java.util.ArrayList cannot be cast to org.bson.Document
Set<String> keysNested = dboNested.keySet();
System.out.println("KeyNested: " + keysNested);




3 Answers
3

Like you've commented, on your code, you cannot convert ArrayList to Document, therefore you must cast it to an ArrayList like below:

while(iterator.hasNext())
String key = iterator.next().toString();
Object value = dbo.get(key);
if (value instanceof ArrayList)
ArrayList<?> dboArrayNested = (ArrayList<?>) value;
for (Object dboNestedObj : dboArrayNested)
if (dboNestedObj instanceof Document)
printKeysNested(Document.class.cast(dboNestedObj));


// else if extra...
else if (value instanceof Document)
printKeysNested(Document.class.cast(value));



public static void printKeysNested(Document dboNested)
Set<String> keysNested = dboNested.keySet();
System.out.println("KeyNested: " + keysNested);



I hope this helps you.

I guess i am late here, but i was trying a more mongo way to do this. You would definitely want to execute this on a small set of query, may be a match stage before even starting this

match

db.coll.aggregate([
$unwind: '$config'
,
$project:
arr:
$objectToArray: '$config'


,
$unwind: '$arr'
,
$group:
_id:
key: '$arr.k'


,
$project:
key: '$_id.key',
_id: 0


])


thanks to all the answers, here is the final code I have:

while (iterator.hasNext())
String key = iterator.next().toString();
Object value = dbo.get(key);
if (value instanceof ArrayList)
ArrayList<?> dboArrayNested = (ArrayList<?>) value;
for (Object dboNestedObj : dboArrayNested)
Document dboNested = Document.class.cast(dboNestedObj);
if (dboNestedObj instanceof Document)
keysNested.addAll(dboNested.keySet());


// else if extra...
else if (value instanceof Document)
Document dboNested = Document.class.cast(value);
keysNested.addAll(dboNested.keySet());


System.out.println("KeysNested: " + keysNested);


this outputs for the given example:
[property1, property2, property3, property4, ispropert5, ispropert6]

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