Spelling suggestion based on bigram field

spelling.json

{
  "suggest": {
    "text": "jeferson memorial",
    "suggestion": {
      "phrase": {
        "analyzer": "bigram_analyzer",
        "field": "bigram",
        "size": 1,
        "direct_generator": [
          {
            "field": "bigram",
            "prefix_len": 1
          }
        ],
        "highlight": {
          "pre_tag": "<strong>",
          "post_tag": "<\/strong>"
        }
      }
    }
  }
}