Extract values from an array in mongoDB to dataframe using rmongodb - arrays

Question

I'm querying a database containing entries as displayed in the example. All entries contain the following values:
_id: unique id of overallitem and placed_items
name: the name of te overallitem
loc: location of the overallitem and placed_items
time_id: time the overallitem was stored
placed_items: array containing placed_items (can range from zero: placed_items : [], to unlimited amount.
category_id: the category of the placed_items
full_id: the full id of the placed_items
I want to extract the name, full_id and category_id on a per placed_items level given a time_id and loc constraint
Example data:
{
"_id" : "5040",
"name" : "entry1",
"loc" : 1,
"time_id" : 20121001,
"placed_items" : [],
}
{
"_id" : "5041",
"name" : "entry2",
"loc" : 1,
"time_id" : 20121001,
"placed_items" : [
{
"_id" : "5043",
"category_id" : 101,
"full_id" : 901,
},
{
"_id" : "5044",
"category_id" : 102,
"full_id" : 902,
}
],
}
{
"_id" : "5042",
"name" : "entry3",
"loc" : 1,
"time_id" : 20121001,
"placed_items" : [
{
"_id" : "5045",
"category_id" : 101,
"full_id" : 903,
},
],
}
The expected outcome for this example would be:
"name" "full_id" "category_id"
"entry2" 901 101
"entry2" 902 102
"entry3" 903 101
So if placed_items is empty, do put the entry in the dataframe and if placed_items containts n entries, put n entries in dataframe
I tried to work out an RBlogger example to create the desired dataframe.
#Set up database
mongo <- mongo.create()
#Set up condition
buf <- mongo.bson.buffer.create()
mongo.bson.buffer.append(buf, "loc", 1)
mongo.bson.buffer.start.object(buf, "time_id")
mongo.bson.buffer.append(buf, "$gte", 20120930)
mongo.bson.buffer.append(buf, "$lte", 20121002)
mongo.bson.buffer.finish.object(buf)
query <- mongo.bson.from.buffer(buf)
#Count
count <- mongo.count(mongo, "items_test.overallitem", query)
#Note that these counts don't work, since the count should be based on
#the number of placed_items in the array, and not the number of entries.
#Setup Cursor
cursor <- mongo.find(mongo, "items_test.overallitem", query)
#Create vectors, which will be filled by the while loop
name <- vector("character", count)
full_id<- vector("character", count)
category_id<- vector("character", count)
i <- 1
#Fill vectors
while (mongo.cursor.next(cursor)) {
b <- mongo.cursor.value(cursor)
order_id[i] <- mongo.bson.value(b, "name")
product_id[i] <- mongo.bson.value(b, "placed_items.full_id")
category_id[i] <- mongo.bson.value(b, "placed_items.category_id")
i <- i + 1
}
#Convert to dataframe
results <- as.data.frame(list(name=name, full_id=full_uid, category_id=category_id))
The conditions work and the code works if I would want to extract values on an overallitem level (i.e. _id or name) but fails to gather the information on a placed_items level. Furthermore, the dotted call for extracting full_id and category_id does not seem to work. Can anyone help?