Control Search Results with Weights¶

This document describes how to create a text index with specified weights for results fields.

By default, the text command returns matching documents based on scores, from highest to lowest. For a text index, the weight of an indexed field denotes the significance of the field relative to the other indexed fields in terms of the score. The score for a given word in a document is derived from the weighted sum of the frequency for each of the indexed fields in that document.

The default weight is 1 for the indexed fields. To adjust the weights for the indexed fields, include the weights option in the db.collection.ensureIndex() method.

Warning

Choose the weights carefully in order to prevent the need to reindex.

A collection blog has the following documents:

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{ _id: 1,
  content: "This morning I had a cup of coffee.",
  about: "beverage",
  keywords: [ "coffee" ]
}

{ _id: 2,
  content: "Who doesn't like cake?",
  about: "food",
  keywords: [ "cake", "food", "dessert" ]
}

To create a text index with different field weights for the content field and the keywords field, include the weights option to the ensureIndex() method. For example, the following command creates an index on three fields and assigns weights to two of the fields:

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db.blog.ensureIndex(
                     {
                       content: "text",
                       keywords: "text",
                       about: "text"
                     },
                     {
                       weights: {
                                  content: 10,
                                  keywords: 5,
                                },
                       name: "TextIndex"
                     }
                   )

The text index has the following fields and weights:

content has a weight of 10,
keywords has a weight of 5, and
about has the default weight of 1.

These weights denote the relative significance of the indexed fields to each other. For instance, a term match in the content field has:

2 times (i.e. 10:5) the impact as a term match in the keywords field and
10 times (i.e. 10:1) the impact as a term match in the about field.

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