Extract Individual particular element from Nested item in MongoDB - mongodb

Question

I am new to MongoDB.
I have created a collection in MongoDB and stored the following
Q1UsefulStatementsList: [{
Q1UsefulStatement: "Useful Sentence"
Q1ActionsList: [{
Q1Verb: "Verb in the sentence"
Q1NP: "The Noun phrase"
Q1PP: "The Preposition phrase"
}]
}]
Q2UsefulStatementsList: [{
Q2UsefulStatement: "Useful Sentence"
Q2ActionsList: [{
Q2Verb: "Verb in the Sentence"
Q2NP: "The Noun phrase"
Q2PP: "The preposition Phrase"
}]
}]
I need to loop through this collection and get all the Verbs from Q1UsefulStatementsList and Q2UsefulStatementsList.
Ex:
Q1UsefulStatementsList: [{
Q1UsefulStatement: "My dog also likes eating sausage"
Q1ActionsList: [{
Q1Verb: "likes"
Q1NP: "My dog"
Q1PP: "n / a"
}]
} {
Q1UsefulStatement: "The disabling of log helps"
Q1ActionsList: [{
Q1Verb: "disabling"
Q1NP: "disaabling of logs"
Q1PP: "of"
}]
}]
Q2UsefulStatementsList: [{
Q2UsefulStatement: "Log analysis failed"
Q2ActionsList: [{
Q2Verb: "failed"
Q2NP: "Log analysis"
Q2PP: "n / a"
}]
}]
I would like to get 'likes' and 'disabling' as output when I run through Q1UsefulStatementsList.
I have tried it using the code below. But is there a easier way to do these sort of things in MongoDB?
I tried using the 'dot operator' like (Q1UsefulStatementsList.Q1UsefulStatement) but what it gives me is a entire BSON(JSON) object. What I need is actual individual values directly.
Do suggest any easier way if any.
The Java code that I have written to extract the values
if (object.get("Q1UsefulStatementsList") != null) {
BasicDBList qUseStatementList = (BasicDBList)(object.get("Q1UsefulStatementsList"));
for (Object qUsefulStatement: qUseStatementList) {
DBObject tmp = (DBObject) qUsefulStatement;
if (tmp.get("Q1ActionsList") != null) {
BasicDBList qActionsList = (BasicDBList) tmp.get("Q1ActionsList");
for (Object qVerbs: qActionsList) {
DBObject tmpQVerbs = (DBObject) qVerbs;
String verb = (tmpQVerbs.get("Q1Verb").toString());
String nP = (tmpQVerbs.get("Q1NP").toString());
String pP = (tmpQVerbs.get("Q1PP").toString());
}
}
}
}

Answer 1

You can use Aggregation Framework to get the verbs you want. You can do it with the following code.
db.myObject.aggregate(
{ $unwind : "$Q1UsefulStatementsList"},
{ $unwind : "$Q1UsefulStatementsList.Q1ActionsList"},
{$group:{ _id: "$_id", verbs : {$addToSet : "$Q1UsefulStatementsList.Q1ActionsList.Q1Verb"}}}
);
This will return results as follow :
"result" : [
{
"_id" : ObjectId("5253e46ae3a2c44e082642c9"),
"verbs" : ["disabling", "likes"]
}
]
By looping the result array you can extract verbs array and add it to the Set in order to get unique verbs.
You can easily convert it to Java code.