gistfile1.txt

"attr": {
    "type": "nested",
    "properties": {
        "key": {
            "type": "string",
            "index": "not_analyzed"
        },
        "text": {            
            "type": "string"
        },
        "integer": {
            "type": "integer"
        },
        "float": {
            "type": "float"
        },
        "date": {
            "type": "date"
        }
    }
}

The mapping allows to store and query user defined values in ES. See example below. You should map the keys in a other databse system to the column name. (ID123 = "User1CustomField", ID456 = "User2CustomField")

{
    "attr" : [
        {
            "key": "ID123",
            "text": "Some text"
        },
        {
            "key": "ID456",
            "integer": 6
        }
    ]
}

you need to query the mapping with nested queries or filters. See
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-nested-type.html
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-nested-query.html
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-nested-filter.html

Double check your business requirements, since this really makes things complex.