More like this query

The More Like This Query finds documents that are “like” a given set of documents. In order to do so, MLT selects a set of representative terms of these input documents, forms a query using these terms, executes the query and returns the results. The user controls the input documents, how the terms should be selected and how the query is formed.

The simplest use case consists of asking for documents that are similar to a provided piece of text. Here, we are asking for all movies that have some text similar to “Once upon a time” in their “title” and in their “description” fields, limiting the number of selected terms to 12.

GET /_search
{
  "query": {
    "more_like_this" : {
      "fields" : ["title", "description"],
      "like" : "Once upon a time",
      "min_term_freq" : 1,
      "max_query_terms" : 12
    }
  }
}

A more complicated use case consists of mixing texts with documents already existing in the index. In this case, the syntax to specify a document is similar to the one used in the Multi GET API.

GET /_search
{
  "query": {
    "more_like_this": {
      "fields": [ "title", "description" ],
      "like": [
        {
          "_index": "imdb",
          "_id": "1"
        },
        {
          "_index": "imdb",
          "_id": "2"
        },
        "and potentially some more text here as well"
      ],
      "min_term_freq": 1,
      "max_query_terms": 12
    }
  }
}

Finally, users can mix some texts, a chosen set of documents but also provide documents not necessarily present in the index. To provide documents not present in the index, the syntax is similar to artificial documents.

GET /_search
{
  "query": {
    "more_like_this": {
      "fields": [ "name.first", "name.last" ],
      "like": [
        {
          "_index": "marvel",
          "doc": {
            "name": {
              "first": "Ben",
              "last": "Grimm"
            },
            "_doc": "You got no idea what I'd... what I'd give to be invisible."
          }
        },
        {
          "_index": "marvel",
          "_id": "2"
        }
      ],
      "min_term_freq": 1,
      "max_query_terms": 12
    }
  }
}

How it Works

Suppose we wanted to find all documents similar to a given input document. Obviously, the input document itself should be its best match for that type of query. And the reason would be mostly, according to Lucene scoring formula, due to the terms with the highest tf-idf. Therefore, the terms of the input document that have the highest tf-idf are good representatives of that document, and could be used within a disjunctive query (or OR) to retrieve similar documents. The MLT query simply extracts the text from the input document, analyzes it, usually using the same analyzer at the field, then selects the top K terms with highest tf-idf to form a disjunctive query of these terms.

The fields on which to perform MLT must be indexed and of type text or keyword` . Additionally, when using like with documents, either _source must be enabled or the fields must be stored or store term_vector. In order to speed up analysis, it could help to store term vectors at index time.

For example, if we wish to perform MLT on the “title” and “tags.raw” fields, we can explicitly store their term_vector at index time. We can still perform MLT on the “description” and “tags” fields, as _source is enabled by default, but there will be no speed up on analysis for these fields.

PUT /imdb
{
  "mappings": {
    "properties": {
      "title": {
        "type": "text",
        "term_vector": "yes"
      },
      "description": {
        "type": "text"
      },
      "tags": {
        "type": "text",
        "fields": {
          "raw": {
            "type": "text",
            "analyzer": "keyword",
            "term_vector": "yes"
          }
        }
      }
    }
  }
}

Parameters

The only required parameter is like, all other parameters have sensible defaults. There are three types of parameters: one to specify the document input, the other one for term selection and for query formation.

Document Input Parameters

Term Selection Parameters

Query Formation Parameters

Alternative

To take more control over the construction of a query for similar documents it is worth considering writing custom client code to assemble selected terms from an example document into a Boolean query with the desired settings. The logic in more_like_this that selects “interesting” words from a piece of text is also accessible via the TermVectors API. For example, using the termvectors API it would be possible to present users with a selection of topical keywords found in a document’s text, allowing them to select words of interest to drill down on, rather than using the more “black-box” approach of matching used by more_like_this.