Retrieve one record

Runs the sample. Generated for this page; it reads the connection string from the environment.

using ArxSamples.PostgreSql;
using ArxSamples.PostgreSql.Operations.Retrieve.Basics;

// The connection string comes from the environment, never from the code (see "Configure the connection").
var connectionString = Environment.GetEnvironmentVariable("ARX_SAMPLES_POSTGRESQL")
    ?? throw new InvalidOperationException("Set the ARX_SAMPLES_POSTGRESQL environment variable to your PostgreSQL connection string.");

await new Basic1_RetrieveOneRecord().RunAsync(new SampleContext(connectionString));

The sample, as it is in the repository.

using ARX.SDK.PostgreSql.Services;
using ARX.SDK.Services;

using static ArxSamples.PostgreSql.SampleOutput;

namespace ArxSamples.PostgreSql.Operations.Retrieve.Basics;

/// <summary>
/// The smallest read: one record, in three scenarios. The usual way first (the id CreateAsync returned, and the columns
/// you want), then a text id, then an id nobody has: Retrieve returns null, not an error.
/// </summary>
public sealed class Basic1_RetrieveOneRecord : ISample
{
    public string Name => "retrieve-basics-1-retrieve-one-record";

    public string Title => "Retrieve one record";

    public async Task RunAsync(SampleContext dbContext)
    {
        await Scenario1_ByTheIdCreateReturnedAsync(dbContext.ConnectionString);

        await Scenario2_ATextIdAsync(dbContext.ConnectionString);

        await Scenario3_NotFoundIsNullAsync(dbContext.ConnectionString);
    }

    private static ArxPgDbService Service(string connectionString) => new ArxPgDbService(connectionString, ArxSchemaCatalog.Create()
        .Table("account", ArxIdColumn.Generated<long>("id"))
        .Table("country", ArxIdColumn.Supplied<string>("code"))
        .Build());

    /// <summary>1. The usual way: the table, the id Create returned, and the columns to read.</summary>
    private static async Task Scenario1_ByTheIdCreateReturnedAsync(string connectionString)
    {
        Step("1. By the id Create returned");

        var service = Service(connectionString);

        var account = new ArxEntity("account");
        account["name"] = "Northwind Traders";
        account["account_number"] = "ACC-R101";
        account["employee_count"] = 120;
        ArxRecordId id = await service.CreateAsync(account);

        ArxEntity read = await service.RetrieveAsync("account", id, new ArxColumnSet("name", "employee_count")) ?? throw new InvalidOperationException("The account record isn't there.");

        // The id is always read too, so read.RecordId can update or delete this record later.
        Show($"{read["name"]}, {read["employee_count"]} employees, RecordId {read.RecordId}");

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * SELECT name, employee_count
         * FROM account
         * WHERE account_number = 'ACC-R101';
         * -- Expect: 1 row, Northwind Traders, 120: what Retrieve returned.
         * ===========================================================
         */
    }

    /// <summary>2. A text id: a country by its code. Without columns, every column is read.</summary>
    private static async Task Scenario2_ATextIdAsync(string connectionString)
    {
        Step("2. A text id");

        var service = Service(connectionString);

        var iceland = new ArxEntity("country");
        iceland["code"] = "IS";
        iceland["name"] = "Iceland";
        await service.CreateAsync(iceland);

        ArxEntity read = await service.RetrieveAsync("country", "IS") ?? throw new InvalidOperationException("The country record isn't there.");

        Show($"{read["code"]} is {read["name"]} ({read.Attributes.Count} columns)");

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * SELECT code, name
         * FROM country
         * WHERE code = 'IS';
         * -- Expect: 1 row, IS, Iceland.
         * ===========================================================
         */
    }

    /// <summary>3. An id that doesn't exist: null, so "is it there?" needs no try/catch.</summary>
    private static async Task Scenario3_NotFoundIsNullAsync(string connectionString)
    {
        Step("3. An id nobody has");

        var service = Service(connectionString);

        ArxEntity? read = await service.RetrieveAsync("account", 999_999_999L);

        Show($"{(read == null ? "null" : "found")}");

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * SELECT count(*)
         * FROM account
         * WHERE id = 999999999;
         * -- Expect: 0.
         * ===========================================================
         */
    }
}

What every sample is given: the connection string (and the ISample interface the samples implement).

namespace ArxSamples.PostgreSql;

/// <summary>One documentation sample: a scenario a real application has, shown end to end.</summary>
public interface ISample
{
    /// <summary>The name to run it alone: <c>dotnet run -- &lt;name&gt;</c>.</summary>
    string Name { get; }

    /// <summary>One line on what it shows.</summary>
    string Title { get; }

    Task RunAsync(SampleContext dbContext);
}

/// <summary>
/// What a sample is given. Every sample gets the connection string; the application's service and catalog
/// (SampleContext.Application.cs) and the plain Npgsql read-back helper (SampleContext.Direct.cs) live in their own
/// files, so a sample needs only the files it uses.
/// </summary>
public sealed partial class SampleContext
{
    public SampleContext(string connectionString)
    {
        ConnectionString = connectionString;
    }

    public string ConnectionString { get; }
}

How the samples print: Step for each scenario, Show for what it did, ShowError for an error caught on purpose.

using ARX.SDK;

namespace ArxSamples.PostgreSql;

/// <summary>How the samples print: a numbered step for each scenario, then what it did.</summary>
public static class SampleOutput
{
    /// <summary>The start of a scenario: <c>Step("1. By the id Create returned")</c>.</summary>
    public static void Step(string text) => Console.WriteLine($"  > {text}");

    /// <summary>What the scenario did.</summary>
    public static void Show(string text) => Console.WriteLine($"      {text}");

    /// <summary>An ARX error the scenario caught on purpose.</summary>
    public static void ShowError(ArxException exception)
        => Console.WriteLine($"      caught {exception.GetType().Name}: {exception.Message}");
}

  > 1. By the id Create returned
      Northwind Traders, 120 employees, RecordId 1
  > 2. A text id
      IS is Iceland (2 columns)
  > 3. An id nobody has
      null