app.rb

src =  [
  {key: 'key1', value: 'value1', data: 'data1'},
  {key: 'key2', value: 'value2', data: 'data2'},
  {key: 'key3', value: 'value3', data: 'data3'}]

part = [
  {key: 'key3', data: 'new_data3'},
  {key: 'key2', value: 'new_value2'}]


#target = [
# {key: 'key1', value: 'value1', data: 'data1'},
# {key: 'key2', value: 'new_value2', data: 'data2'},
# {key: 'key3', value: 'value3', data: 'new_data3'}]

def merge(src, part)
  target = src.dup
  part.each { |part_h| target.detect { |target_h| target_h[:key] == part_h[:key] }&.merge!(part_h) }
  target
end