Ruby on Rails导出到csv-维护mysql select语句顺序

Question

Exporting some data from mysql to a csv file using FasterCSV. I'd like the columns in the outputted CSV to be in the same order as the select statement in my query.

Example:

rows = Data.find(
  :all,
  :select=>'name, age, height, weight'
)

headers = rows[0].attributes.keys
FasterCSV.generate do |csv|
  csv << headers
  rows.each do |r|
    csv << r.attributes.values
  end
end

CSV Output:

height,weight,name,age
74,212,bob,23
70,201,fred,24
.
.
.

I want the CSV columns in the same order as my select statement. Obviously the attributes method is not going to work. Any ideas on the best way to ensure that the columns in my csv file will be in the same order as the select statement? Got a lot of data and performance is an issue. The select statement is not static. I realize I could loop through column names within the rows.each loop but it seems kinda dirty.

Answer 1

Use the Comma gem:

class Data < ActiveRecord:Base

  comma do
    name
    age
    height
    weight
  end

  comma :height_weight do
   name
   age
   height_in_feet
   weight
 end


end

Now you can generate the CSV as follows:

Data.all(:select => 'name, age, height, weight').to_comma

Data.all(:select => 'name, age, height_in_feet, weight').to_comma(:height_weight)

Edit:

The ActiveRecord finders does not support calculated columns in the resultset, ie

data = Data.first(:select => 'name, age, height/12 as height_in_feet, weight')
data.height_in_feet # throws error

You can use select_extra_columns gem if you want to include the calculated columns.

Answer 2

Try this:

def export_to_csv (rows, col_names)
  col_names = col_names.split(",") if  col_names.is_a?(String)
  FasterCSV.generate do |csv|
    # header row
    csv << col_names

    # data rows
    rows.each do |row|
      csv << col_names.collect{|name| row.send(name)}
    end
  end
end

cols = 'name, age, height, weight'
rows = Data.find(:all, :select=> cols)
csv = export_to_csv(rows, cols)