Find a document (C#)

Finds a single document in a collection using filter and sort clauses, including vector search.

Ready to write code? See the examples for this method to get started. If you are new to the Data API, check out the quickstart.

Result

Returns a document that matches the specified filter and sort clauses, or returns null if no document was found.

The fields included in the returned document depend on the subset of fields that were requested in the projection. If requested and applicable, the document will also include a $similarity key with a numeric similarity score that represents the closeness of the sort vector and the document’s vector.

Parameters

Use the FindOneAsync method, which belongs to the Collection class. You can also use FindOne, which is the synchronous version of the method.

Method signature

public Task<T> FindOneAsync(
  CollectionFindOneOptions<T> options = null
);

public Task<T> FindOneAsync(
  CollectionFilter<T> filter,
  CollectionFindOneOptions<T> options = null
);

public Task<TResult> FindOneAsync<TResult>(
  CollectionFindOneOptions<T> options = null
) where TResult : class;

public Task<TResult> FindOneAsync<TResult>(
  CollectionFilter<T> filter,
  CollectionFindOneOptions<T> options = null
) where TResult : class;

Name Type Summary

Name	Type	Summary
`filter`	`CollectionFilter`	An object that defines filter criteria using the Data API filter syntax. The method only finds documents that match the filter criteria. Filters can improve performance by reducing the number of documents that the Data API processes. You must use `&` to escape any `.` or `&` in field names in the filter clause. You cannot use `&` to escape any other characters. For more information, see Work with `.` and `&` in field names (C#). For a list of available filter operators and more examples, see Filter operators for collections (C#). Filters can use only indexed fields. If you apply selective indexing when you create a collection, you cannot reference non-indexed fields in a filter.
`options`	`CollectionFindOneOptions`	Optional. Options for this operation. For more information and examples for general options such as timeout, see Customize API interaction. For options specific to this method, see Method-specific properties of the `CollectionFindOneOptions` class.

filter

CollectionFilter

An object that defines filter criteria using the Data API filter syntax. The method only finds documents that match the filter criteria. Filters can improve performance by reducing the number of documents that the Data API processes.

You must use & to escape any . or & in field names in the filter clause. You cannot use & to escape any other characters. For more information, see Work with . and & in field names (C#).

For a list of available filter operators and more examples, see Filter operators for collections (C#).

Filters can use only indexed fields. If you apply selective indexing when you create a collection, you cannot reference non-indexed fields in a filter.

options

CollectionFindOneOptions

Optional. Options for this operation. For more information and examples for general options such as timeout, see Customize API interaction. For options specific to this method, see Method-specific properties of the CollectionFindOneOptions class.

Method-specific properties of the `CollectionFindOneOptions` class
Name	Type	Summary
`Projection`	`IProjectionBuilder`	Optional. Controls which fields are included or excluded in the returned document. You must use `&` to escape any `.` or `&` in field names in the projection clause. You cannot use `&` to escape any other characters. For more information, see Work with `.` and `&` in field names (C#). For more information, see Projections for collections (C#). Default: The default projection for the collection. All fields prefixed with `$` are excluded by default and will only be returned if you include them in the projection. `_id` is included by default and will always be returned unless you exclude it from the projection.
`IncludeSimilarity`	`bool`	Optional. Whether to include a `$similarity` property in the response. The `$similarity` value represents the closeness of the sort vector and the document’s vector. This parameter only applies if you use a vector search. Default: false
`Sort`	`SortBuilder<T>`	Optional. Sorts documents by one or more fields, or performs a vector search. You must use `&` to escape any `.` or `&` in field names in the sort clause. You cannot use `&` to escape any other characters. For more information, see Work with `.` and `&` in field names (C#). For more information, see Sort clauses for collections (C#). Sort clauses can use only indexed fields. If you apply selective indexing when you create a collection, you cannot reference non-indexed fields in sort queries. For vector searches, this parameter can use `$vector`.

Examples

The following examples demonstrate how to find a document in a collection.

Use a document’s ID to find a document

All documents have a unique _id property. You can use a filter to find a document with a specific _id.

The following example uses untyped documents, but you can use strongly-typed classes for compile-time checks and IntelliSense. For more information and examples, see Custom typing for collections.

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find a document
    var filter = Builders<Document>.CollectionFilter.Eq("_id", "101");
    var result = await collection.FindOneAsync(filter);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Use filters to find a document

You can use a filter to find a document that matches specific criteria. For example, you can find a document with an is_checked_out value of false and a number_of_pages value less than 300.

For a list of available filter operators and more examples, see Filter operators for collections (C#).

Filters can use only indexed fields. If you apply selective indexing when you create a collection, you cannot reference non-indexed fields in a filter.

The following example uses untyped documents, but you can use strongly-typed classes for compile-time checks and IntelliSense. For more information and examples, see Custom typing for collections.

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find a document
    var filterBuilder = Builders<Document>.CollectionFilter;
    var filter = filterBuilder.And(
      filterBuilder.Eq("is_checked_out", false),
      filterBuilder.Lt("number_of_pages", 300)
    );

    var result = await collection.FindOneAsync(filter);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Use vector search to find a document

To find the document whose $vector value is most similar to a given vector, use a sort with the vector embeddings that you want to match. For more information, see Find data with vector search.

Vector search is only available for vector-enabled collections. For more information, see Create a collection that can store vector embeddings and $vector in collections (C#).

The following example uses untyped documents, but you can use strongly-typed classes for compile-time checks and IntelliSense. For more information and examples, see Custom typing for collections.

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;
using DataStax.AstraDB.DataApi.Core.Query;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find a document
    var embeddings = new float[] { 0.08f, -0.62f, 0.39f };
    var findOptions = new CollectionFindOneOptions<Document>()
    {
      Sort = Builders<Document>.CollectionSort.Vector(embeddings),
    };

    var result = await collection.FindOneAsync(findOptions);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Use lexicographical matching to find a document

Lexicographical matching is currently in public preview. Development is ongoing, and the features and functionality are subject to change. Hyper-Converged Database (HCD), and the use of such, is subject to the DataStax Preview Terms.

There are two ways to use lexicographical matching to find documents with the Data API:

Sort on the $lexical field to find the documents whose $lexical field value is most relevant to a given string of space-separated keywords or terms
Filter on the $lexical field with the $match operator to find the documents whose $lexical field value is a lexicographical match to the specified string of space-separated keywords or terms

You can use these strategies together or separately.

You can only use lexicographical matching on collections that have lexical enabled. For more information, see Create a collection that supports lexicographical matching.

Documents must have the $lexical field populated to be included in lexicographical matching. For examples, see Insert a document for retrieval with lexicographical matching and Insert documents for retrieval with lexicographical matching.

The following example uses untyped documents, but you can use strongly-typed classes for compile-time checks and IntelliSense. For more information and examples, see Custom typing for collections.

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;
using DataStax.AstraDB.DataApi.Core.Query;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find a document
    var filter = Builders<Document>.CollectionFilter.LexicalMatch(
      "tree hill"
    );

    var findOptions = new CollectionFindOneOptions<Document>()
    {
      Sort = Builders<Document>.CollectionSort.Lexical(
        "tree hill grassy"
      ),
    };

    var result = await collection.FindOneAsync(filter, findOptions);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Use sorting to find a document

You can use a sort clause to sort documents by one or more fields.

For more information, see Sort clauses for collections (C#).

Sort clauses can use only indexed fields. If you apply selective indexing when you create a collection, you cannot reference non-indexed fields in sort queries.

The following example uses untyped documents, but you can use strongly-typed classes for compile-time checks and IntelliSense. For more information and examples, see Custom typing for collections.

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;
using DataStax.AstraDB.DataApi.Core.Query;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find a document
    var findOptions = new CollectionFindOneOptions<Document>()
    {
      Sort = Builders<Document>
        .CollectionSort.Ascending("rating")
        .Descending("title"),
    };

    var result = await collection.FindOneAsync(findOptions);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Include only specific fields in the response

To specify which fields to include or exclude in the returned document, use a projection.

All fields prefixed with $ are excluded by default and will only be returned if you include them in the projection. _id is included by default and will always be returned unless you exclude it from the projection.

The following example uses untyped documents, but you can use strongly-typed classes for compile-time checks and IntelliSense. For more information and examples, see Custom typing for collections.

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;
using DataStax.AstraDB.DataApi.Core.Query;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find a document
    var filterBuilder = Builders<Document>.CollectionFilter;
    var filter = filterBuilder.Eq("metadata.language", "English");
    var findOptions = new CollectionFindOneOptions<Document>()
    {
      Projection = Builders<Document>
        .Projection.Include("is_checked_out")
        .Include("title"),
    };

    var result = await collection.FindOneAsync(filter, findOptions);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Exclude specific fields from the response

To specify which fields to include or exclude in the returned document, use a projection.

The following example uses untyped documents, but you can use strongly-typed classes for compile-time checks and IntelliSense. For more information and examples, see Custom typing for collections.

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;
using DataStax.AstraDB.DataApi.Core.Query;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find a document
    var filterBuilder = Builders<Document>.CollectionFilter;
    var filter = filterBuilder.Eq("metadata.language", "English");
    var findOptions = new CollectionFindOneOptions<Document>()
    {
      Projection = Builders<Document>
        .Projection.Exclude("is_checked_out")
        .Exclude("title"),
    };

    var result = await collection.FindOneAsync(filter, findOptions);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Use filter, sort, and projection together

The following example uses untyped documents, but you can use strongly-typed classes for compile-time checks and IntelliSense. For more information and examples, see Custom typing for collections.

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;
using DataStax.AstraDB.DataApi.Core.Query;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find a document
    var filterBuilder = Builders<Document>.CollectionFilter;
    var filter = filterBuilder.And(
      filterBuilder.Eq("is_checked_out", false),
      filterBuilder.Lt("number_of_pages", 300)
    );

    var findOptions = new CollectionFindOneOptions<Document>()
    {
      Sort = Builders<Document>
        .CollectionSort.Ascending("rating")
        .Descending("title"),
      Projection = Builders<Document>
        .Projection.Include("is_checked_out")
        .Include("title"),
    };

    var result = await collection.FindOneAsync(filter, findOptions);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Work with `.` and `&` in field names

You must use & to escape any . or & in field names when the field is used in a filter, sort, projection, update, or indexing clause. Dot notation, which is used to reference nested fields, should not be escaped. For more information, see Work with . and & in field names (C#).

For example, in the following document, you would use escaping like this: areas.r&&d, costs.price&.usd, and costs.price&.cad.

{
  "areas": {
    "r&d": true,
    "design": false
  },
  "costs": {
    "price.usd": 100,
    "price.cad": 90
  }
}

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;
using DataStax.AstraDB.DataApi.Core.Query;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find documents
    var filterBuilder = Builders<Document>.CollectionFilter;
    var filter = filterBuilder.And(
      filterBuilder.Eq("areas.r&&d", false),
      filterBuilder.Lt("costs.price&.usd", 300)
    );

    var findOptions = new CollectionFindOneOptions<Document>()
    {
      Sort = Builders<Document>.CollectionSort.Ascending(
        "costs.price&.usd"
      ),
      Projection = Builders<Document>
        .Projection.Include("areas.r&&d")
        .Include("costs.price&.cad"),
    };

    var result = await collection.FindOneAsync(filter, findOptions);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

You can also use the EscapeFieldNames function provided by the client:

using System.Text.Json;
using DataStax.AstraDB.DataApi;
using DataStax.AstraDB.DataApi.Collections;
using DataStax.AstraDB.DataApi.Core;
using DataStax.AstraDB.DataApi.Core.Query;
using DataStax.AstraDB.DataApi.Utils;

namespace Examples;

public class Program
{
  static async Task Main()
  {
    // Get an existing collection
    var client = new DataAPIClient(
      new CommandOptions() { Destination = DataAPIDestination.HCD }
    );

    var database = client.GetDatabase(
      "API_ENDPOINT",
      DataAPIClient.UsernamePasswordTokenProvider(
        "USERNAME",
        "PASSWORD"
      ),
      "KEYSPACE_NAME"
    );

    var collection = database.GetCollection("COLLECTION_NAME");

    // Find documents
    var filterBuilder = Builders<Document>.CollectionFilter;
    var filter = filterBuilder.And(
      filterBuilder.Eq(
        FieldEscaping.EscapeFieldNames("areas", "r&d"),
        false
      ),
      filterBuilder.Lt(
        FieldEscaping.EscapeFieldNames("costs", "price.usd"),
        300
      )
    );

    var findOptions = new CollectionFindOneOptions<Document>()
    {
      Sort = Builders<Document>.CollectionSort.Ascending(
        FieldEscaping.EscapeFieldNames("costs", "price.usd")
      ),
      Projection = Builders<Document>
        .Projection.Include(
          FieldEscaping.EscapeFieldNames("areas", "r&d")
        )
        .Include(FieldEscaping.EscapeFieldNames("costs", "price.cad")),
    };

    var result = await collection.FindOneAsync(filter, findOptions);

    Console.WriteLine(JsonSerializer.Serialize(result));
  }
}

Client reference

For more information, see the client reference.

Find a document (C#)

Result

Parameters

Examples

Use a document’s ID to find a document

Use filters to find a document

Use vector search to find a document

Use lexicographical matching to find a document

Use sorting to find a document

Include only specific fields in the response

Exclude specific fields from the response

Use filter, sort, and projection together

Work with `.` and `&` in field names

Client reference

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