Thesaurus synonyms

Defining a thesaurus

A thesaurus is a tool used to organize and categorize words based on their meanings and relationships. It can be built using the ThesaurusBuilder class, which allows you to add both synonyms, hyponyms and hypernyms.

WithSynonyms(ICollection<string> synonyms)

The WithSynonyms method is used to instruct the thesaurus to expand any of the given synonyms when one of them indexed. For example, if you add the synonyms dog, doggy, puppy, and pup to the thesaurus and include the terms "I love my doggy" and "We got a new puppy today" in the index, a search for "dog" would return both of these entries because doggy and puppy are synonymous with dog.

WithHyponyms(string word, ICollection<string> hyponyms)

The WithHyponyms method is used to instruct the thesaurus to expand the search to include any words that are more specific or narrower in meaning compared to the given term. These narrower terms are known as hyponyms.

For example, if you add the hyponyms poodle and beagle to the thesaurus for the term dog using:

WithHyponyms("dog", "poodle", "beagle");

• searching for "dog" would return entries that contain poodle and beagle because they are hyponyms of dog.
• searching for "poodle" or "beagle" will return only entries that explicitly contain those terms - other mentions of dog, e.g. “Dogs are a type of domestic animal” will not be returned

WithHypernyms(string word, ICollection<string> hypernyms)

The WithHypernyms method is used to instruct the thesaurus to expand the search to include any words that are more general or broader in meaning compared to the given terms. These broader terms are known as hypernyms.

For example, if you add the hypernyms mammal and canine to the thesaurus for the term dog:

WithHypernyms("dog", "mammal", "canine");

• searching for "mammal" would return entries that contain dog because mammal is a hypernym of dog.
• searching for "dog" will return only entries that explicitly contain dog - other mentions of mammal, e.g. “Mammals are warm blooded” will not be returned

Default thesaurus

FullTextIndexBuilder<TKey> WithDefaultThesaurus(Func<ThesaurusBuilder, ThesaurusBuilder> thesaurusBuilder)

Builds the default thesaurus to use for the index. This thesaurus will be used for text added directly to the index and also fields that have no explicit thesaurus defined for them.

var index = new FullTextIndexBuilder<int>()
    .WithDefaultThesaurus(o => o.WithHyponyms("dog", "poodle", "beagle"))
    .Build();

Field-level thesaurus

A thesaurus can be configured specifically for an object’s field - in this example the hyponyms will only be applied to the Content field:

var index = new FullTextIndexBuilder<int>()
    .WithObjectTokenization<Book>(
    o => o
        .WithKey(i => i.Id)
        .WithField("Content", i => i.Content, thesaurusOptions: o => o.WithHyponyms("dog", "poodle", "beagle")
        .WithField("Title", i => i.Title));

Interplay between the thesaurus and tokenizer

The values in a thesaurus are automatically passed through the tokenizer for the relevant field. For example, an index constructed as:

var index = new FullTextIndexBuilder<int>()
    .WithDefaultTokenizer(o => o.CaseInsensitive())
    .WithDefaultThesaurus(o => o.WithSynonyms("conduct", "conduction"))
    .WithObjectTokenization<Book>(
    o => o
        .WithKey(i => i.Id)
        .WithField("Content", i => i.Content, tokenizationOptions: o => o.WithStemming())
        .WithField("Title", i => i.Title, tokenizationOptions: o => o.CaseInsensitive(false)));

Would end up with 3 variations of the thesaurus values:

• Once for the default tokenizer with case insensitivity applied
• Once for the “Content” with stemming applied to the words
• Once for the “Title” field without case insensitivity applied