this is a solution to export big query multiple csv files into one mongo collection

gs2mongo.sh

# Requirements: script should be ran on relevant mongo instance
# Input: <google storage path> <db_name> <collection_name>
# TL;DR: loads multiple csv files in to mongo collection
# Author: Tal Peretz

# init vars
gs_path=$1
db_name=$2
collection_name=$3
temp_csv_name=partial_csv
files=`gsutil ls $gs_path` # get all partial csv files
progress_file=gs2mongo_finished_files_list.txt

# touch helper files
touch $progress_file
touch $temp_csv_name

# iterate files in gs_path
    for f in $files
      do
        if grep -Fxq "$f" $progress_file
        then
	        echo "$f is already imported to mongo"
        else
          gsutil cp $f $temp_csv_name # copy partial file locally
          mongoimport -d $db_name -c $collection_name --type csv --file "$temp_csv_name" --headerline # import partial file to mongo
          echo "finished importing $f to $collection_name"
          echo "$f" >> $progress_file
          rm -f $temp_csv_name
        fi
      done