rmongodb：如何通过在投影查询中指定来强制执行字段排序？

Question

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?

Answer 1

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()