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2. Use Operators & Collectors

moreLikeThis

On this page

  • Definition
  • Behavior
  • Usage
  • Syntax
  • Options
  • Limitations
  • Examples

Definition

moreLikeThis

The moreLikeThis operator returns documents similar to input documents. The moreLikeThis operator allows you to build features for your applications that display similar or alternative results based on one or more given documents.

Behavior

When you run a moreLikeThis query, Atlas Search performs these actions:

  • Extracts a limited number of most representative terms based on the input documents that you specify in the operator's like option.

  • Creates a disjunction (OR) query to find similar documents based on the most representative terms and returns the results.

The moreLikeThis operator performs a search for similar documents using the analyzer that you specify in the index configuration. If you omit the analyzer in the index definition, the moreLikeThis operator uses the default standard analyzer. If you specify multiple analyzers, the moreLikeThis operator runs the input text through each analyzer, searches, and returns results for all analyzers.

To view the disjunction (OR) that Atlas Search constructs to find similar documents, use explain with your moreLikeThis operator query.

Usage

Before you can run the moreLikeThis operator query, we recommend that you retrieve one or more input documents. To retrieve input documents, you can do one of the following:

  • Run a query, such as find(), or another MQL query to find BSON documents.

  • Run any aggregation pipeline that returns BSON documents.

  • Use any other source of documents in your application.

Once you identify the input documents, you can pass them to the moreLikeThis operator.

When you run a moreLikeThis operator query, Atlas Search returns the original input document in the query results. To omit the input document from the query results, use the moreLikeThis operator in a compound operator query and exclude the input document by its _id using the equals operator in the mustNot clause.

Syntax

moreLikeThis has the following syntax:

 {
   "$search": {
     "index": index name, // optional, defaults to "default"
     "moreLikeThis": {
       "like": [
         {
           <"field-name">: <"field-value">,
           ...
         },
         ...
       ],
       "score": <options>
     }
   }
 }

Options

moreLikeThis uses the following option to constuct a query:

Field
Type
Description
Necessity
like
one BSON document or an array of documents
One or more BSON documents that Atlas Search uses to extract representative terms to query for.
Required
score
object

Score to assign to matching search results. You can modify the default score using the following options:

  • boost: multiply the result score by the given number.

  • constant: replace the result score with the given number.

  • function: replace the result score with the given expression.

For information on using score in your query, see Score the Documents in the Results.

When you query values in arrays, Atlas Search doesn't alter the score of the matching results based on the number of values inside the array that matched the query. The score would be the same as a single match regardless of the number of matches inside an array.

Optional

Limitations

You can't use the moreLikeThis operator to query non-string values. To search for non-string values, you can combine a moreLikeThis query with a near, range, or any other operator in a compound operator query.

You can't use the moreLikeThis operator inside the embeddedDocument operator to query documents in an array.

Examples

The examples use the movies collection in the sample_mflix database. Each example in this section uses a different index definition to demonstrate different features of the operator.

Before you run the example queries on your cluster, load the sample data on your Atlas cluster, and create the suggested index. To learn more about creating an Atlas Search index using the UI, API, or CLI, see Create an Atlas Search Index. The index definitions use the name default.

Note

If you name your index default, you don't need to specify an index parameter when using the $search pipeline stage. Otherwise, you must specify the index name using the index parameter.

Example 1: Single Document with Multiple Fields

The following example uses the moreLikeThis operator to find documents that are similar to multiple field values. For this example, the index defition contains dynamic mappings to dynamically index all dynamically indexable field types in the collection. Your index definition for the sample_mflix.movies collection should look similar to the following.

{
  "mappings": {
    "dynamic": true
  }
}

Example

The following query searches for movies that are similar to the input movie title "The Godfather" and input movie genre "action". It includes a $limit stage to limit the output to 5 results and a $project stage to exclude all fields except title, released, and genres.

1db.movies.aggregate([
2  {
3    "$search": {
4    moreLikeThis: {
5     like:
6      {
7        "title": "The Godfather",
8        "genres": "action"
9      }
10    }
11   }
12  },
13  { "$limit": 5},
14  {
15    $project: {
16      "_id": 0,
17      "title": 1,
18      "released": 1,
19      "genres": 1
20    }
21  }
22])
[
 { genres: [ 'Comedy', 'Drama', 'Romance' ], title: 'Godfather' },
 {
  genres: [ 'Crime', 'Drama' ],
  title: 'The Godfather',
  released: ISODate("1972-03-24T00:00:00.000Z")
 },
 {
  genres: [ 'Crime', 'Drama' ],
  title: 'The Godfather: Part II',
  released: ISODate("1974-12-20T00:00:00.000Z")
 },
 {
  genres: [ 'Crime', 'Drama' ],
  title: 'The Godfather: Part III',
  released: ISODate("1990-12-26T00:00:00.000Z")
 },
 {
  genres: [ 'Action' ],
  title: 'The Defender',
  released: ISODate("1994-07-28T00:00:00.000Z")
 }
]

Atlas Search results contain movies similar to the input movie title "The Godfather" and input movie genre "action".

Example 2: Input Document Excluded in Results

The following example uses find() to identify an input document, and then uses the moreLikeThis operator to find similar documents. For this example, the index definition uses static mappings to index only the title, genres, and _id fields.

1{
2  "mappings": {
3    "dynamic": false,
4    "fields": {
5      "title": {
6        "type": "string"
7      },
8      "genres": {
9        "type": "string"
10      },
11      "_id": {
12        "type": "objectId"
13      }
14    }
15  }
16}

Example

The following find() query finds the movie with the title "The Godfather" and stores the result inside movie. It specifies that the results should only contain the title and genres fields for the matching documents. Note that, by default, the find () command always returns the _id field, the value for which might be different on your cluster.

movie = db.movies.find( { title: "The Godfather" }, { genres: 1, title: 1} ).toArray()
[
 {
  _id: ObjectId("573a1396f29313caabce4a9a"),
  genres: [ 'Crime', 'Drama' ],
  title: 'The Godfather'
 }
]

The following query uses a compound operator with the moreLikeThis operator to query the title and genres fields and the equals operator to exclude the input document using the following clauses:

  • The must clause to query for movies similar to the movie stored in movie.

  • The mustNot clause to exclude the input document from results by its _id value. Note that the _id value used in the query matches the _id value in the results of the preceding find() query.

The query limits the output to 5 results. The query uses a $project stage to include the _id, title, released, and genres fields in the results.

Note

Before you run this query, replace the value of the _id field on line 13 with the value of the _id field in your query results.

1db.movies.aggregate([
2  {
3    "$search": {
4       "compound":{
5          "must":[{
6            "moreLikeThis": {
7              "like": movie
8            }
9          }],
10          "mustNot":[{
11            "equals": {
12               "path": "_id",
13               "value": ObjectId ("573a1396f29313caabce4a9a")
14            }
15          }]
16         }
17       }
18     },
19{"$limit": 5},
20{
21 "$project": {
22   "_id": 1,
23   "title": 1,
24   "released": 1,
25   "genres": 1
26  }
27 }
28])
[
 {
  _id: ObjectId("573a13acf29313caabd27afc"),
  genres: [ 'Comedy', 'Drama', 'Romance' ],
  title: 'Godfather'
 },
 {
  _id: ObjectId("573a1396f29313caabce557f"),
  genres: [ 'Crime', 'Drama' ],
  title: 'The Godfather: Part II',
  released: ISODate("1974-12-20T00:00:00.000Z")
 },
 {
  _id: ObjectId("573a1398f29313caabcebf7b"),
  genres: [ 'Crime', 'Drama' ],
  title: 'The Godfather: Part III',
  released: ISODate("1990-12-26T00:00:00.000Z")
 },
 {
  _id: ObjectId("573a1399f29313caabceed8d"),
  genres: [ 'Action' ],
  title: 'The Defender',
  released: ISODate("1994-07-28T00:00:00.000Z")
 },
 {
  _id: ObjectId("573a139af29313caabcef2a0"),
  genres: [ 'Action' ],
  title: 'The Enforcer',
  released: ISODate("1995-03-02T00:00:00.000Z")
 }
]

Atlas Search results include documents that are similar to the query term The Godfather in the action genre. However, the results don't include the document that was excluded by its _id, which is ObjectId("573a1396f29313caabce4a9a").

Example 3: Multiple Analyzers

The following example uses find() to identify input documents, and then uses a moreLikeThis operator to find similar documents. For this example, the index definition uses static mappings to index the fields in the sample_mflix.movies collection with different analyzers. The index definition:

  • Configures an index on the _id, title and genres fields.

  • Analyzes the title field using the lucene.standard analyzer and an alternate analyzer named keywordAnalyzer that uses the lucene.keyword analyzer.

  • Analyzes and searches the fields using the lucene.english analyzer.

1{
2  "mappings": {
3    "dynamic": false,
4    "fields": {
5      "title": {
6        "type": "string",
7        "analyzer": "lucene.standard",
8        "multi": {
9          "keywordAnalyzer": {
10            "type": "string",
11            "analyzer": "lucene.keyword"
12          }
13        }
14      },
15      "genres": {
16        "type": "string"
17      },
18      "_id": {
19        "type": "objectId"
20      }
21    }
22  },
23  "analyzer": "lucene.english"
24}

Example

The following find() query retrieves the movies with the title "Alice in Wonderland" and stores the results in movie. It specifies that the results should only contain the title and genres fields for the matching documents. Note that, by default, the find() command always returns the _id field, the value for which might be different on your cluster.

movie = db.movies.find( { title: "Alice in Wonderland" }, { genres: 1, title: 1} ).toArray
[
  {
   _id: ObjectId("573a1394f29313caabcde9ef"),
   plot: 'Alice stumbles into the world of Wonderland. Will she get home? Not if the Queen of Hearts has her way.',
   title: 'Alice in Wonderland'
  },
  {
   _id: ObjectId("573a1398f29313caabce963d"),
   plot: 'Alice is in Looking Glass land, where she meets many Looking Glass creatures and attempts to avoid the Jabberwocky, a monster that appears due to her being afraid.',
   title: 'Alice in Wonderland'
  },
  {
   _id: ObjectId("573a1398f29313caabce9644"),
   plot: 'Alice is in Looking Glass land, where she meets many Looking Glass creatures and attempts to avoid the Jabberwocky, a monster that appears due to her being afraid.',
   title: 'Alice in Wonderland'
  },
  {
   _id: ObjectId("573a139df29313caabcfb504"),
   plot: `The wizards behind The Odyssey (1997) and Merlin (1998) combine Lewis Carroll's "Alice in Wonderland" and "Through the Looking Glass" into a two-hour special that just gets curiouser and curiouser.`,
   title: 'Alice in Wonderland'
  },
  {
   _id: ObjectId("573a13bdf29313caabd5933b"),
   plot: "Nineteen-year-old Alice returns to the magical world from her childhood adventure, where she reunites with her old friends and learns of her true destiny: to end the Red Queen's reign of terror.",
   title: 'Alice in Wonderland'
  }
]

The following example uses a compound operator to query the title and genres fields using the following clauses:

  • The should clause uses the moreLikeThis operator to search for documents similar to the document in movie. Note that the title field is analyzed with both the lucene.standard and lucene.keyword analyzers.

  • The mustNot clause specifies that one of the input documents, specified by its _id value, must not be included in the results.

The query limits the results list to 10 documents. The query uses a $project stage to include the _id, title, and genres fields in the results.

Example

1db.movies.aggregate([
2{
3   $search: {
4     "compound": {
5       "should": [{
6          "moreLikeThis": {
7             "like": movie
8          }
9       }],
10     "mustNot": [
11     {
12     "equals": {
13     "path": "_id",
14      "value": ObjectId ("573a1394f29313caabcde9ef")
15    }
16   }]
17 }
18  }
19  },
20 { $limit: 10 },
21          {
22 $project: {
23     "title": 1,
24     "genres": 1,
25     "_id": 1
26   }
27  }
28 ])
[
  {
   _id: ObjectId("573a1398f29313caabce963d"),
   genres: [ 'Adventure', 'Family', 'Fantasy' ],
   title: 'Alice in Wonderland'
  },
  {
   _id: ObjectId("573a1398f29313caabce9644"),
   genres: [ 'Adventure', 'Family', 'Fantasy' ],
   title: 'Alice in Wonderland'
  },
  {
   _id: ObjectId("573a139df29313caabcfb504"),
   genres: [ 'Adventure', 'Comedy', 'Family' ],
   title: 'Alice in Wonderland'
  },
  {
   _id: ObjectId("573a13bdf29313caabd5933b"),
   genres: [ 'Adventure', 'Family', 'Fantasy' ],
   title: 'Alice in Wonderland'
  },
  {
   _id: ObjectId("573a1396f29313caabce3e7e"),
   genres: [ 'Comedy', 'Drama' ],
   title: 'Alex in Wonderland'
  },
  {
   _id: ObjectId("573a13bdf29313caabd5a44b"),
   genres: [ 'Drama' ],
   title: 'Phoebe in Wonderland'
  },
  {
   _id: ObjectId("573a139af29313caabcf0e23"),
   genres: [ 'Documentary' ],
   title: 'Wonderland'
  },
  {
   _id: ObjectId("573a139ef29313caabcfcebc"),
   genres: [ 'Drama' ],
   title: 'Wonderland'
  },
  {
   _id: ObjectId("573a13a0f29313caabd03dab"),
   genres: [ 'Drama' ],
   title: 'Wonderland'
  },
  {
   _id: ObjectId("573a13abf29313caabd2372a"),
   genres: [ 'Crime', 'Drama', 'Mystery' ],
   title: 'Wonderland'
  }
]

The following query uses explain with the preceding query to show the disjunction (OR) that Atlas Search constructs to find similar documents.

db.movies.explain("queryPlanner").aggregate([
 {
   $search: {
     "compound": {
       "should": [{
          "moreLikeThis": {
             "like": [{
                "title": "Alice in Wonderland"
             }]
          }
       }],
     "mustNot": [
     {
     "equals": {
     "path": "_id",
      "value": ObjectId ("573a1394f29313caabcde9ef")
    }
   }]
 }
  }
  },
{ $limit: 10 },
          {
$project: {
     "title": 1,
     "genres": 1,
     "_id": 1
   }
 }
])
{
 explainVersion: '1',
   stages: [
     {
      '$_internalSearchMongotRemote': {
         mongotQuery: {
         compound: {
         should: [
          {
            moreLikeThis: { like: [ { title: 'Alice in Wonderland' } ] }
          }
         ],
         mustNot: [
          {
            equals: {
              path: '_id',
              value: ObjectId("573a1394f29313caabcde9ef")
            }
          }
         ]
      }
     },
     explain: {
      type: 'BooleanQuery',
        args: {
         must: [],
         mustNot: [
           {
             path: 'compound.mustNot',
             type: 'ConstantScoreQuery',
             args: {
               query: {
               type: 'TermQuery',
             args: {
                path: '_id',
                value: '[57 3a 13 94 f2 93 13 ca ab cd e9 ef]'
               }
              }
            }
           }
         ],
         should: [
          {
            path: 'compound.should',
            type: 'BooleanQuery',
            args: {
             must: [],
             mustNot: [],
             should: [
              {
               type: 'TermQuery',
               args: { path: 'title', value: 'in' }
              },
              {
                type: 'TermQuery',
                args: {
                  path: 'title.keywordAnalyzer',
                  value: 'Alice in Wonderland'
                }
               },
              {
                type: 'TermQuery',
                args: { path: 'title', value: 'wonderland' }
              },
              {
                type: 'TermQuery',
                args: { path: 'title', value: 'alice' }
              }
             ],
             filter: [],
              minimumShouldMatch: 0
             }
            }
          ],
            filter: [],
             minimumShouldMatch: 0
            }
           }
          }
         },
 { '$_internalSearchIdLookup': {} },
 { '$limit': Long("10") },
 { '$project': { _id: true, title: true, genres: true } }
  ],
 serverInfo: {
 ...
 },
 serverParameters: {
 ...
 },
 command: {
   aggregate: 'movies',
     pipeline: [
       {
        '$search': {
           compound: {
             should: [
               {
                 moreLikeThis: { like: [ { title: 'Alice in Wonderland' } ] }
               }
             ],
             mustNot: [
              {
                equals: {
                  path: '_id',
                  value: ObjectId("573a1394f29313caabcde9ef")
                }
              }
             ]
           }
          }
        },
  { '$limit': 10 },
  { '$project': { title: 1, genres: 1, _id: 1 } }
  ],
  cursor: {},
  '$db': 'sample_mflix'
  },
  ok: 1,
  '$clusterTime': {
  clusterTime: Timestamp({ t: 1659133479, i: 1 }),
  signature: {
     hash: Binary(Buffer.from("865d9ef1187ae1a74c4a0da1e29882aebcf2be7c", "hex"), 0),
     keyId: Long("7123262728533180420")
  }
 },
 operationTime: Timestamp({ t: 1659133479, i: 1 })
 }

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