jsonArray = JSON.parse(File.open(JSON_filePath).read)
jsonArray << node_to_insert

File.open(JSON_FilePath,"w") do |f|
   f.write(JSON.pretty_generate(jsonArray))
end

jsonArray = [node_to_insert] # initialize to array with your first row
jsonArray.push( *JSON.parse(File.open(JSON_filePath).read) ) # push results,
               # use the splat operator (*) to avoid array nesting

jsonArray = JSON.parse(File.open(JSON_filePath).read)
jsonArray.prepend(node_to_insert)