How do I use functions like CONCAT(), etc. in ARel?

问题

Is there a way to have ARel write (sanitized, possibly aliased, etc.) column names into CONCAT() and other SQL functions?

Here's how to do it with AVG()...

?> name = Arel::Attribute.new(Arel::Table.new(:countries), :name)
=> #<struct Arel::Attributes::Attribute [...]
?> population = Arel::Attribute.new(Arel::Table.new(:countries), :population)
=> #<struct Arel::Attributes::Attribute [...]
?> Country.select([name, population.average]).to_sql
=> "SELECT `countries`.`name`, AVG(`countries`.`population`) AS avg_id FROM `countries`"

(yes, I know that avg_id would be the same in every row, just trying to illustrate my question)

So what if I wanted a different function?

?> Country.select(xyz).to_sql # Arel::Concat.new(name, population) or something?
=> "SELECT CONCAT(`countries`.`name`, ' ', `countries`.`population`) AS concat_id FROM `countries`"

Thanks!

回答1:

Use NamedFunction:

name = Arel::Attribute.new(Arel::Table.new(:countries), :name)
func = Arel::Nodes::NamedFunction.new 'zomg', [name]
Country.select([name, func]).to_sql

来源：https://stackoverflow.com/questions/9304724/how-do-i-use-functions-like-concat-etc-in-arel