I have some data I am trying to retrieve from mongodb into R using rmongodb
package. At some point in time, the ordering of the fields in the stored documents changed.
I am trying to force my projection query to keep the order of the fields projected fixed by explicitly specifying approach attempted in: SO question as follows:
data <- mongo.find.all(mongo_conn, table,
fields = list('id1' = 1, 'id2' = 2,
'time' = 3, 'latitude' = 4,
'longitude' = 5, '_id' = 0))
I can't seem to find a good answer to this. It returns the fields in the order they are in the DB, which changed, as a list of course.
That means, it wreaks obvious havoc in what kind of loopy code I have to write to organize the returned results into a data frame like structure.
Any idea how I can get the fields in specified order and not what is in DB?
In the answer you link to it says
Simple answer is that you can't do this.
Also see the related mongodb ticket
However, to get your results in a data.frame
-like structure, use mongolite
, it's much easier to work with
Consider this example using the mtcars data
data("mtcars")
library(mongolite)
mongo <- mongo(db = "test",
collection = "mtcars",
url = "mongodb://localhost")
## insert into database
mongo$insert(mtcars)
# Complete! Processed total of 32 rows.
# [1] TRUE
mongolite::find
will automatically simplify the query results into a data.frame structure
df_results <- mongo$find()
# Imported 32 records. Simplifying into dataframe...
head(df_results)
# mpg cyl disp hp drat wt qsec vs am gear carb
# Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
# Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
# Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1
# Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
# Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2
# Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1
Or, using the aggregation framework
mongo$aggregate(pipeline = '[{ "$project" : { "mpg" : 1, "wt" : 1, "_id" : 0} },
{ "$limit" : 5 }]')
# Imported 5 records. Simplifying into dataframe...
# mpg wt
# 1 21.0 2.620
# 2 21.0 2.875
# 3 22.8 2.320
# 4 21.4 3.215
# 5 18.7 3.440
And now for a bit of shameless self-promotion. I've been working on an extension to mongolite that returns a data.table object. The idea here is to increase the speed of returned objects, but only if the returned result set can be coerced using rbindlist
.
The package is mongolitedt
, and still in development.
# devtools::install_github("SymbolixAU/mongolitedt")
library(mongolitedt)
bind_mongolitedt(mongo)
mongo$aggregatedt(pipeline = '[{ "$project" : { "mpg" : 1, "wt" : 1, "_id" : 0} },
{ "$limit" : 5 }]')
## now have a data.table object returned
# Imported 5 records.
# mpg wt
# 1: 21.0 2.620
# 2: 21.0 2.875
# 3: 22.8 2.320
# 4: 21.4 3.215
# 5: 18.7 3.440
## clean up
rm(mongo); gc()
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