Ruby CSV - Write on same row without overwriting? - ruby

Question

I'm using
CSV.open(filename, "w") do |csv|
to create and write to a csv file in one ruby.rb file and now I need to open it and edit it in a second .rb file. Right now I'm using CSV.open(filename, "a") do |csv| but that creates new rows rather than adding the new content to the end of the existing rows.
If I use CSV.open(filename, "w") do |csv| the second time it overwrites the first rows.
edit:
# Create export CSV
final_export_csv = "filepath_final.csv"
# Create filename for CSV file
imported_csv_filename = "imported_file.csv"
CSV.open(final_export_csv, "w", headers: ["several", "headers"] + [:new_header], write_headers: true) do |final_csv|
# Read existing CSV file
CSV.foreach(imported_csv_filename) do |old_csv_row|
# Read a row, add the new column, write it to the new row
CSV.open(denominator_csv_filename, "r+") do |new_csv_col|
# gathering some data code
data = { passed.in }
# Write data
new_csv_col <<
[
passedin[:data]
]
old_csv_row[:new_header] = passedin[:data]
final_export_csv << old_csv_row
end
end
end
end
end

Answer 1

As tadman comments, you can't actually edit a file in place. Well, you can but all the lines have to remain the same length. You're not doing that.
Instead, read a row, modify it, and write it to a new CSV. Then replace the old file with the new one. Be careful to avoid slurping the entire CSV into memory, CSV files can get quite large.
require 'csv'
require 'tempfile'
require 'fileutils'
csv_file = "test.csv"
# Write the new file to a tempfile to avoid polluting the directory.
temp = Tempfile.new
# Read the header line.
old_csv = CSV.open(csv_file, "r", headers: true, return_headers: true)
old_csv.readline
# Open the new CSV with the existing headers plus a new one.
new_csv = CSV.open(
temp, "w",
headers: old_csv.headers + [:new],
write_headers: true
)
# Read a row, add the new column, write it to the new CSV.
old_csv.each do |row|
row[:new] = 42
new_csv << row
end
old_csv.close
new_csv.close
# Replace the old CSV with the new one.
FileUtils.move(temp.path, csv_file)