I use mysql to store user data, and search the data by Elasticsearch. For mysql, I have a user defined field, this field can store a JSON format data. 

the example data like this:

```
data1 = 
{
"name" : "test1",
"age": 10,
"user_defined": {
  "a" : "aaa",
  "b" : "bbb",
  "c" : "ccc",
  .....
  }
}

data2 = 
{
"name" : "test2",
"age": 20,
"user_defined": {
  "d" : "ddd",
  "e" : "eee",
  "f" : "fff",
  .....
  }
}

```

For user\_defined field, the number of keys is not fixed, the type of values all are string, I hope each key can be searched, how to define the mapping? how to search this kind of data by Elasticsearch?

Anyone has good idea?