in NEST, how do I dynamically build a query from a list of terms? - elasticsearch

Question

Say my user provides a list of search terms which I've collected into an array/list, and now I want to combine those OR-wise into a NEST query using MatchPhrase. How would I do that? The code for a (single) search term would look something like this:
var search = client.Search<ElasticRequirement>(s => s
.Query(q =>
q.MatchPhrase(m => m.OnField(f => f.Title).Query(term.ToLower()).Slop(slop))
|| q.MatchPhrase(m => m.OnField(f => f.Description).Query(text).Slop(slop))
)
.LowercaseExpandedTerms()
.Explain()
.Query(q => q.Fuzzy(f => f.PrefixLength(1).OnField(c => c.Title).OnField(c => c.Description)))
);
This is fine, but I need to apply that same MatchPhrase filter once for each provided search term. Any help much appreciated.

Answer 1

You can use bool should expressions to build your query dynamically. I'll provide the complete solution below. Call BuildQuery() method with appropriate parameters.
ISearchResponse<ElasticRequirement> BuildQuery(IElasticClient client, IEnumerable<string> terms, int slop)
{
return client.Search<ElasticRequirement>(s => s
.Query(q => q
.Bool(b => b
.Should(terms.Select(t => BuildPhraseQueryContainer(q, t, slop)).ToArray())))
.LowercaseExpandedTerms()
.Explain()
.Query(q => q.Fuzzy(f => f.PrefixLength(1).OnField(c => c.Title).OnField(c => c.Description))));
}
QueryContainer BuildPhraseQueryContainer(QueryDescriptor<ElasticRequirement> qd, string term, int slop)
{
return qd.MatchPhrase(m => m.OnField(f => f.Title).Query(term.ToLower()).Slop(slop)) ||
qd.MatchPhrase(m => m.OnField(f => f.Description).Query(term.ToLower()).Slop(slop));
}
For terms = {"term1", "term2", "term3"} and slop = 0, the Elasticsearch search JSON command that will get built by my code is as under:
{
"explain": true,
"query": {
"bool": {
"should": [
{
"bool": {
"should": [
{
"match": {
"title": {
"type": "phrase",
"query": "term1",
"slop": 0
}
}
},
{
"match": {
"description": {
"type": "phrase",
"query": "term1",
"slop": 0
}
}
}
]
}
},
{
"bool": {
"should": [
{
"match": {
"title": {
"type": "phrase",
"query": "term2",
"slop": 0
}
}
},
{
"match": {
"description": {
"type": "phrase",
"query": "term2",
"slop": 0
}
}
}
]
}
},
{
"bool": {
"should": [
{
"match": {
"title": {
"type": "phrase",
"query": "term3",
"slop": 0
}
}
},
{
"match": {
"description": {
"type": "phrase",
"query": "term3",
"slop": 0
}
}
}
]
}
}
]
}
}
}
You can tweak this code such that all the match commands are under the same should node. I'll leave that up to you to figure out :)