Can not convert array to string: problem of a newbie

hi guys,

def fil_string
sql = ActiveRecord::Base.connection()
filter_string=’ ’
sql.begin_db_transaction
column_ids = sql.execute(“SELECT columnid FROM output_columns”)
column_ids.each do |column_id|
filter_string=filter_string +" columnid=’"+column_id+"’"
end
sql.commit_db_transaction
end

but in the solution
Error: can not convert array to string
at line – filter_string=filter_string +" columnid=’"+column_id+"’" "

Please take a hand in my code.