Several calls in one ARX transaction: commit or roll back

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

using ArxSamples.MySQL;
using ArxSamples.MySQL.Operations.Create.AdvancedCases;

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

await new Advanced2_ArxTransaction().RunAsync(new SampleContext(connectionString));

The sample, as it is in the repository.

using ARX.SDK;
using ARX.SDK.MySQL.Services;
using ARX.SDK.Services;
using static ArxSamples.MySQL.SampleOutput;

namespace ArxSamples.MySQL.Operations.Create.AdvancedCases;

/// <summary>
/// Several ARX calls in one transaction, with ARX types only, in three
/// scenarios: commit, roll back, and a failed call that aborts the transaction. Open an
/// <see cref="ArxMySqlConnection"/>, begin a transaction on it, pass the transaction to each call, then commit or roll
/// back. No MySqlConnector type appears in application code.
/// </summary>
/// <remarks>
/// Use a transaction when your code decides, step by step, whether the work is kept: create the account, create its
/// contacts, check something, then commit or roll back. For imports in batches and processes ARX runs from start to
/// finish, the batch scope samples (3 and 4) do the same with less code.
/// </remarks>
public sealed class Advanced2_ArxTransaction : ISample
{
    public string Name => "create-advanced-2-arx-transaction";

    public string Title => "Several calls in one ARX transaction: commit or roll back";

    public async Task RunAsync(SampleContext dbContext)
    {
        await Scenario1_CommitAsync(dbContext);

        await Scenario2_RollBackAsync(dbContext);

        await Scenario3_AFailedCallAbortsTheTransactionAsync(dbContext);
    }

    /// <summary>1. Commit: an account and its contacts, kept together.</summary>
    private static async Task Scenario1_CommitAsync(SampleContext dbContext)
    {
        Step("1. Commit: an account and its contacts, kept together");

        var service = dbContext.Service;

        // The connection is ARX's own: it opens on BeginTransactionAsync and closes when disposed.
        await using (var connection = new ArxMySqlConnection(dbContext.ConnectionString))
        await using (var transaction = await connection.BeginTransactionAsync())
        {
            var account = new ArxEntity("account");
            account["name"] = "Woodgrove Bank";
            account["industry"] = new ArxSelectionSetValue("Industry", Industry.Finance);

            // Pass the transaction to each call: the rows are written inside it.
            var accountId = await service.CreateAsync(account, transaction);

            var ines = new ArxEntity("contact");
            ines["account_id"] = new ArxEntityReference("account", accountId);
            ines["first_name"] = "Ines";
            ines["last_name"] = "Moreau";

            var tom = new ArxEntity("contact");
            tom["account_id"] = new ArxEntityReference("account", accountId);
            tom["first_name"] = "Tom";
            tom["last_name"] = "Becker";

            await service.CreateMultipleAsync(new List<ArxEntity> { ines, tom }, transaction);

            // Until the commit, other connections don't see any of it.
            Show($"before commit, other connections see {await dbContext.Direct.CountAsync("account", "name = @p0", "Woodgrove Bank")} Woodgrove accounts");

            await transaction.CommitAsync();
        }

        Show($"after commit: {await dbContext.Direct.CountAsync("account", "name = @p0", "Woodgrove Bank")} account, "
            + $"{await dbContext.Direct.CountAsync("contact", "last_name IN ('Moreau', 'Becker')")} contacts");

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * SELECT a.name, a.industry, c.first_name, c.last_name
         * FROM contact c
         * JOIN account a ON a.id = c.account_id
         * WHERE c.last_name IN ('Moreau', 'Becker');
         * -- Expect: 2 rows, Ines Moreau and Tom Becker of Woodgrove Bank, industry 4 (Finance).
         * ===========================================================
         */
    }

    /// <summary>2. Roll back: your own check fails after the account was created.</summary>
    private static async Task Scenario2_RollBackAsync(SampleContext dbContext)
    {
        Step("2. Roll back: your own check fails after the account was created");

        await using (var connection = new ArxMySqlConnection(dbContext.ConnectionString))
        await using (var transaction = await connection.BeginTransactionAsync())
        {
            var account = new ArxEntity("account");
            account["name"] = "Margie's Travel";
            account["credit_limit"] = 50_000m;
            await dbContext.Service.CreateAsync(account, transaction);

            var creditCheckPassed = false;   // say your credit check service says no

            if (creditCheckPassed)
            {
                await transaction.CommitAsync();
            }
            else
            {
                await transaction.RollbackAsync();
            }

            // Leaving the block without committing would roll back too: disposing an active transaction rolls it back.
        }

        Show($"after rollback: {await dbContext.Direct.CountAsync("account", "name = @p0", "Margie's Travel")} Margie's Travel accounts");

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * SELECT count(*)
         * FROM account
         * WHERE name = 'Margie''s Travel';
         * -- Expect: 0: rolled back.
         * ===========================================================
         */
    }

    /// <summary>
    /// 3. In MySQL, a statement that fails usually leaves the transaction open with the earlier rows in it (some
    /// errors end it), so a later COMMIT would keep half the work. ARX treats every database failure alike: the
    /// transaction is marked aborted, further calls are refused, and CommitAsync rolls back and throws instead of
    /// committing part of it.
    /// </summary>
    private static async Task Scenario3_AFailedCallAbortsTheTransactionAsync(SampleContext dbContext)
    {
        Step("3. A failed call aborts the transaction: ARX refuses to commit it");

        await using (var connection = new ArxMySqlConnection(dbContext.ConnectionString))
        await using (var transaction = await connection.BeginTransactionAsync())
        {
            var account = new ArxEntity("account");
            account["name"] = "Alpine Ski House";
            account["account_number"] = "ACC-40001";
            await dbContext.Service.CreateAsync(account, transaction);

            var duplicate = new ArxEntity("account");
            duplicate["name"] = "Alpine Ski House (copy)";
            duplicate["account_number"] = "ACC-40001";

            try
            {
                await dbContext.Service.CreateAsync(duplicate, transaction);
            }
            catch (ArxDatabaseException error) when (error.Code == ArxErrorCode.UniqueViolation)
            {
                ShowError(error);
                Show($"transaction.IsAborted = {transaction.IsAborted}");
            }

            try
            {
                await transaction.CommitAsync();
            }
            catch (ArxValidationException error) when (error.Code == ArxErrorCode.TransactionAborted)
            {
                ShowError(error);
            }
        }

        Show($"stored: {await dbContext.Direct.CountAsync("account", "name LIKE 'Alpine Ski House%'")} Alpine Ski House accounts");

        /* ===========================================================
         * [DIRECT QUERY CHECK]
         * -----------------------------------------------------------
         * SELECT count(*)
         * FROM account
         * WHERE account_number = 'ACC-40001';
         * -- Expect: 0: the failed call aborted the transaction, so nothing was committed.
         * ===========================================================
         */
    }
}

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

namespace ArxSamples.MySQL;

/// <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 MySqlConnector 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.MySQL;

/// <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}");
}

The application's ARX service and its schema catalog, created once and shared by the samples.

using ARX.SDK.MySQL.Services;
using ARX.SDK.Services;

namespace ArxSamples.MySQL;

/// <summary>The application's ARX service and its schema catalog, shared by the real and advanced samples.</summary>
public sealed partial class SampleContext
{
    private ArxSchemaCatalog? _catalog;
    private ArxMySqlDbService? _service;

    /// <summary>The application's schema catalog: table ids, keys and selection sets (SampleCatalog.cs).</summary>
    public ArxSchemaCatalog Catalog => _catalog ??= SampleCatalog.Build();

    /// <summary>
    /// One service for the whole application. It holds no connection: every call opens one from the MySqlConnector pool and
    /// closes it, unless you pass your own connection or transaction.
    /// </summary>
    public ArxMySqlDbService Service => _service ??= new ArxMySqlDbService(ConnectionString, Catalog);
}

The application's schema catalog: every table's id column and keys, and its selection sets.

using ARX.SDK.Services;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace ArxSamples.MySQL;

/// <summary>An account's industry. Stored in account.industry (SMALLINT) as the enum's number.</summary>
public enum Industry : short
{
    Retail = 1,
    Manufacturing = 2,
    Healthcare = 3,
    Finance = 4,
    Technology = 5
}

/// <summary>
/// The application's schema catalog: every table's id column and keys, and every picklist (selection set),
/// defined once.
/// </summary>
/// <remarks>
/// <para>
/// <b>Ids.</b> Every table the application writes is declared with its one id column: its name, its .NET type, and
/// whether the database generates it (<c>ArxIdColumn.Generated</c>: identity, sequence, default) or the caller supplies
/// it (<c>ArxIdColumn.Supplied</c>: a natural id, a client-generated Guid). A table with no id column, such as a log or a
/// table identified by several columns together, is declared <c>ArxIdColumn.NoIdColumn</c>. ARX never reads ids from
/// the database: the developer knows the schema, and an undeclared table is refused (ARX1016).
/// </para>
/// <para>
/// <b>Keys.</b> Optional business keys: one or more columns whose values identify a record, such as an account number,
/// or a region and a SKU together. Create doesn't use them (the database's unique constraints do the enforcing); Update
/// and Delete will be able to find rows by them. <c>ValidateSchemaAsync</c> checks ids and keys against the database
/// when you ask it to.
/// </para>
/// <para>
/// <b>Selection sets.</b> /// A selection set (picklist, option set) is a column that only accepts a fixed list of codes, each with a display
/// text: an account's status (ACTIVE, ON_HOLD, CLOSED), a contact's preferred channel (E, P, S). The catalog tells ARX,
/// for each set:
/// </para>
/// <list type="bullet">
/// <item><b>Name</b>: what an <see cref="ArxSelectionSetValue"/> refers to, for example <c>new ArxSelectionSetValue("AccountStatus", "ON_HOLD")</c>.</item>
/// <item><b>Kind</b>: what the codes are, matching the column: Integer, String, Character, Boolean, Decimal or Guid.
/// (A fixed list of text values is a String set, whether the column is an ENUM or a VARCHAR that a CHECK guards.)</item>
/// <item><b>Multiplicity</b>: Single (one code), or several codes stored as bit flags (Flags), a delimited string
/// (Delimited) or a JSON array (Json). MySQL has no array columns.</item>
/// <item><b>Options</b>: each code with its display text and order. A retired option (<c>IsActive: false</c>) stays
/// readable in old rows, but can't be written any more.</item>
/// </list>
/// <para>
/// Build the catalog once at startup and give it to the service: <c>new ArxMySqlDbService(connectionString, SampleCatalog.Build())</c>.
/// ARX checks every <see cref="ArxSelectionSetValue"/> against it in memory, before any SQL is sent, and stores the code
/// itself in the column: no lookup table, no extra query. A plain value (a string or number that isn't an
/// ArxSelectionSetValue) isn't checked.
/// </para>
/// <para>
/// The catalog is plain code, so the options can come from wherever you keep them: written out here, taken from a .NET
/// enum (<c>AddEnum</c>), loaded from JSON (<see cref="SetsJson"/> and <see cref="FromJson"/> below), or loaded at
/// startup from your own option table and passed to <c>new ArxSelectionSetDefinition(name, kind, options)</c>. ARX
/// never reads option tables or files itself.
/// </para>
/// </remarks>
public static class SampleCatalog
{
    /// <summary>
    /// Selection sets kept as JSON, as an application might keep them in a file, a settings store or a database column.
    /// Here the JSON is in the class itself; in an application, read it from wherever you keep it
    /// (<c>File.ReadAllText("selection-sets.json")</c>, configuration, an HTTP call) and pass it to <see cref="FromJson"/>.
    /// "multiplicity" and "active" are optional (Single and true).
    /// </summary>
    public const string SetsJson = """
        [
          {
            "name": "LeadSource",
            "kind": "String",
            "options": [
              { "value": "WEB",       "text": "Website",    "order": 0 },
              { "value": "REFERRAL",  "text": "Referral",   "order": 1 },
              { "value": "EVENT",     "text": "Trade show", "order": 2 },
              { "value": "COLD_CALL", "text": "Cold call",  "order": 3, "active": false }
            ]
          },
          {
            "name": "ContactRating",
            "kind": "Integer",
            "multiplicity": "Single",
            "options": [
              { "value": 1, "text": "Poor" },
              { "value": 2, "text": "Fair" },
              { "value": 3, "text": "Good" },
              { "value": 4, "text": "Very good" },
              { "value": 5, "text": "Excellent" }
            ]
          }
        ]
        """;

    public static ArxSchemaCatalog Build()
    {
        var builder = ArxSchemaCatalog.Create()

            // ---------- tables: id column and keys ----------
            // Generated BIGINT ids (AUTO_INCREMENT): the database fills them in. The keys are the unique columns that also
            // identify a record.
            .Table("account", ArxIdColumn.Generated<long>("id")).Key("account", "account_number")
            .Table("contact", ArxIdColumn.Generated<long>("id")).Key("contact", "email")

            // Retrieve: a contact's account_id reads back as a reference to its account, named by the account's
            // name; a contact is named by its first and last names.
            .Lookup("contact", "account_id", "account")
            .DisplayName("account", "name")
            .DisplayName("contact", new[] { "first_name", "last_name" }, " ")

            .Table("product_category", ArxIdColumn.Generated<long>("id")).Key("product_category", "name")
            .Table("product", ArxIdColumn.Generated<long>("id")).Key("product", "sku")
            .Table("sales_order", ArxIdColumn.Generated<long>("id")).Key("sales_order", "order_number")
            .Table("order_line", ArxIdColumn.Generated<long>("id"))
            .Table("data_type_showcase", ArxIdColumn.Generated<long>("id"))

            // A natural id: the caller gives the ISO country code.
            .Table("country", ArxIdColumn.Supplied<string>("code"))

            // A Guid id the application gives (CHAR(36)): MySQL can only hand back generated integer ids.
            .Table("device", ArxIdColumn.Supplied<Guid>("id")).Key("device", "serial_number")

            // No id column: a price is identified by its region and SKU together, a compound key.
            .Table("price_list", ArxIdColumn.NoIdColumn).Key("price_list", "region", "sku")

            // No primary key at all: the same action can be logged twice.
            .Table("activity_log", ArxIdColumn.NoIdColumn)

            // A table without an id: a visit is a fact, never updated (Retrieve Advanced8).
            .Table("web_visit", ArxIdColumn.NoIdColumn)

            // ---------- selection sets ----------

            // From a .NET enum: the set is named after the enum ("Industry"), the codes are its numbers (Retail = 1, …),
            // the display texts are its member names. Column: account.industry SMALLINT.
            .AddEnum<Industry>()

            // Written out: String codes with display texts, single choice. Column: account.status VARCHAR(10).
            .Add("AccountStatus", ArxSelectionSetKind.String, ("ACTIVE", "Active"), ("ON_HOLD", "On hold"), ("CLOSED", "Closed"))

            // Multiple choice, stored as delimited text ("ENTERPRISE,PUBLIC"). RESELLER is retired: rows that have it still
            // read back, but new writes can't use it. Column: account.segments VARCHAR(200).
            .Add(new ArxSelectionSetDefinition("CustomerSegment", ArxSelectionSetKind.String, new[]
            {
                new ArxSelectionSetOption("SMB", "Small business", 0),
                new ArxSelectionSetOption("ENTERPRISE", "Enterprise", 1),
                new ArxSelectionSetOption("PUBLIC", "Public sector", 2),
                new ArxSelectionSetOption("RESELLER", "Reseller", 3, IsActive: false)
            }, ArxSelectionSetMultiplicity.Delimited))

            // Single-character codes. Column: contact.preferred_channel CHAR(1).
            .Add("ContactChannel", ArxSelectionSetKind.Character, ('E', "Email"), ('P', "Phone"), ('S', "Text message"))

            // A fixed list of text values, in a MySQL ENUM column, which allows only its labels.
            // Column: data_type_showcase.mood ENUM('happy', 'neutral', 'sad').
            .Add(new ArxSelectionSetDefinition("Mood", ArxSelectionSetKind.String, new[]
            {
                new ArxSelectionSetOption("happy", "Happy", 0),
                new ArxSelectionSetOption("neutral", "Neutral", 1),
                new ArxSelectionSetOption("sad", "Sad", 2)
            }));

        // Loaded from JSON: LeadSource (contact.lead_source VARCHAR(50)) and ContactRating (contact.rating SMALLINT).
        foreach (var set in FromJson(SetsJson))
        {
            builder.Add(set);
        }

        return builder.Build();
    }

    /// <summary>Reads selection set definitions from JSON (the shape of <see cref="SetsJson"/>).</summary>
    public static IEnumerable<ArxSelectionSetDefinition> FromJson(string json)
    {
        var options = new JsonSerializerOptions
        {
            PropertyNameCaseInsensitive = true,
            Converters = { new JsonStringEnumConverter() }   // "String", "Integer", "Array", … as names
        };

        var sets = JsonSerializer.Deserialize<List<JsonSet>>(json, options)
            ?? throw new InvalidOperationException("The selection set JSON is empty.");

        foreach (var set in sets)
        {
            yield return new ArxSelectionSetDefinition(
                set.Name,
                set.Kind,
                set.Options.Select(x => new ArxSelectionSetOption(Code(x.Value, set.Kind), x.Text, x.Order, x.Active)),
                set.Multiplicity,
                set.DatabaseTypeName);
        }
    }

    /// <summary>A JSON value as a code of the set's kind: JSON has no char, Guid or 64-bit integer of its own.</summary>
    private static object Code(JsonElement value, ArxSelectionSetKind kind) => kind switch
    {
        ArxSelectionSetKind.Integer => value.GetInt64(),
        ArxSelectionSetKind.Decimal => value.GetDecimal(),
        ArxSelectionSetKind.Boolean => value.GetBoolean(),
        ArxSelectionSetKind.Guid => value.GetGuid(),
        ArxSelectionSetKind.Character => value.GetString()![0],
        _ => value.GetString()!   // String and DatabaseEnum
    };

    private sealed record JsonSet(
        string Name,
        ArxSelectionSetKind Kind,
        List<JsonOption> Options,
        ArxSelectionSetMultiplicity Multiplicity = ArxSelectionSetMultiplicity.Single,
        string? DatabaseTypeName = null);

    private sealed record JsonOption(JsonElement Value, string Text, int Order = 0, bool Active = true);
}

The plain MySqlConnector read-back helper, for the samples that show what MySQL stored.

namespace ArxSamples.MySQL;

/// <summary>plain MySqlConnector outside the SDK, for the samples that show what MySQL stored.</summary>
public sealed partial class SampleContext
{
    private MySqlDirectHelper? _direct;

    /// <summary>plain MySqlConnector outside the SDK, used only to reset the tables and to show what was stored.</summary>
    public MySqlDirectHelper Direct => _direct ??= new MySqlDirectHelper(ConnectionString);
}

Plain MySqlConnector, not part of ARX: reads rows back so a sample can show what MySQL stored.

using MySqlConnector;

namespace ArxSamples.MySQL;

/// <summary>
/// Plain MySqlConnector, outside ARX.SDK: resets the sample tables, reads rows back so a sample can show what MySQL
/// actually stored (printing is SampleOutput.cs). It isn't part of the SDK and isn't how an application uses ARX; it only
/// lets the samples verify and show their results.
/// </summary>
public sealed class MySqlDirectHelper
{
    private readonly string _connectionString;

    public MySqlDirectHelper(string connectionString)
    {
        _connectionString = connectionString;
    }

    /// <summary>The same helper pointed at another database on the same server.</summary>
    public MySqlDirectHelper ForDatabase(string database) => new MySqlDirectHelper(ConnectionStringFor(database));

    /// <summary>The connection string of <see cref="ForDatabase"/>'s database, for a second ARX service.</summary>
    public string ConnectionStringFor(string database)
        => new MySqlConnectionStringBuilder(_connectionString) { Database = database }.ConnectionString;

    /// <summary>Creates <paramref name="database"/> on the server if it doesn't exist yet (connecting without a database).</summary>
    public async Task EnsureDatabaseExistsAsync(string database)
    {
        await using var connection = new MySqlConnection(ConnectionStringFor(string.Empty));
        await connection.OpenAsync();

        // CREATE DATABASE takes no parameters; the name is the sample's own constant, quoted.
        await using var create = new MySqlCommand($"CREATE DATABASE IF NOT EXISTS `{database.Replace("`", "``")}`", connection);
        await create.ExecuteNonQueryAsync();
    }

    /// <summary>
    /// Drops and recreates the sample tables (Schema/sample-schema.sql), statement by statement, as the mysql client runs
    /// it (procedure bodies between DELIMITER lines). The optional reporting database is skipped for a user who can't create it.
    /// </summary>
    public async Task ResetSchemaAsync()
    {
        await EnsureDatabaseExistsAsync(new MySqlConnectionStringBuilder(_connectionString).Database);
        var script = await File.ReadAllTextAsync(Path.Combine(AppContext.BaseDirectory, "Schema", "sample-schema.sql"));

        var skippedReporting = false;

        foreach (var statement in ScriptStatements(script))
        {
            if (skippedReporting && statement.Contains(REPORTING_DATABASE, StringComparison.Ordinal))
            {
                continue;
            }

            try
            {
                await ExecuteAsync(statement);
            }
            catch (MySqlException e) when (statement.Contains(REPORTING_DATABASE, StringComparison.Ordinal)
                && e.Number is 1044 or 1142 or 1227) // access denied to a database, a table, or a privilege
            {
                // The script's optional part: a user who can't create a database goes on without it.
                skippedReporting = true;
                Console.WriteLine($"Skipped the optional {REPORTING_DATABASE} database ({e.Message}); only RetrieveMultiple's Advanced 7 reads it.");
            }
        }
    }

    private const string REPORTING_DATABASE = "arx_samples_reporting";

    /// <summary>
    /// The statements of a mysql client script: each ends with the current delimiter (<c>;</c>, or what a
    /// <c>DELIMITER</c> line sets, so a procedure's body keeps its own semicolons). Comment lines never end a statement.
    /// </summary>
    public static IEnumerable<string> ScriptStatements(string script)
    {
        var delimiter = ";";
        var current = new System.Text.StringBuilder();

        foreach (var raw in script.Split('\n'))
        {
            var line = raw.TrimEnd('\r');
            var trimmed = line.Trim();

            if (trimmed.StartsWith("DELIMITER ", StringComparison.OrdinalIgnoreCase))
            {
                delimiter = trimmed.Substring("DELIMITER ".Length).Trim();
                continue;
            }

            current.AppendLine(line);

            if (!trimmed.StartsWith("--", StringComparison.Ordinal) && trimmed.EndsWith(delimiter, StringComparison.Ordinal))
            {
                var text = current.ToString().TrimEnd();
                current.Clear();
                yield return text.Substring(0, text.Length - delimiter.Length);
            }
        }

        if (current.ToString().Split('\n').Any(x => x.Trim().Length > 0 && !x.Trim().StartsWith("--", StringComparison.Ordinal)))
        {
            yield return current.ToString();
        }
    }

    public async Task ExecuteAsync(string sql)
    {
        await using var connection = await OpenConnectionAsync();
        await using var command = new MySqlCommand(sql, connection);
        await command.ExecuteNonQueryAsync();
    }

    public async Task<MySqlConnection> OpenConnectionAsync()
    {
        var connection = new MySqlConnection(_connectionString);
        await connection.OpenAsync();
        return connection;
    }

    /// <summary>
    /// Counts committed rows, optionally with a WHERE clause using @p0, @p1, … for <paramref name="parameters"/>. A plain
    /// SELECT is InnoDB's consistent read: it shows what is committed without waiting for another transaction's locks.
    /// </summary>
    public async Task<long> CountAsync(string table, string where = "1 = 1", params object[] parameters)
    {
        await using var connection = await OpenConnectionAsync();
        await using var command = new MySqlCommand($"SELECT COUNT(*) FROM {table} WHERE {where}", connection);

        for (var i = 0; i < parameters.Length; i++)
        {
            command.Parameters.AddWithValue("@p" + i, parameters[i] ?? DBNull.Value);
        }

        return Convert.ToInt64(await command.ExecuteScalarAsync());
    }

    /// <summary>Reads one row by id as column → value (null for SQL NULL).</summary>
    public async Task<Dictionary<string, object?>> RowAsync(string table, long id)
    {
        await using var connection = await OpenConnectionAsync();
        await using var command = new MySqlCommand($"SELECT * FROM {table} WHERE id = @p0", connection);
        command.Parameters.AddWithValue("@p0", id);
        await using var reader = await command.ExecuteReaderAsync();

        if (!await reader.ReadAsync())
        {
            throw new InvalidOperationException($"No {table} row with id {id}.");
        }

        var row = new Dictionary<string, object?>(StringComparer.OrdinalIgnoreCase);

        for (var i = 0; i < reader.FieldCount; i++)
        {
            row[reader.GetName(i)] = reader.IsDBNull(i) ? null : reader.GetValue(i);
        }

        return row;
    }

    /// <summary>
    /// Reads columns of one row as MySQL prints them as text, in a UTC session (<c>CAST(column AS CHAR)</c>; binary values
    /// as hex), which shows the stored value exactly, whatever its type: a TIMESTAMP as its UTC time.
    /// </summary>
    public async Task<Dictionary<string, string?>> TextRowAsync(string table, long id, params string[] columns)
    {
        await using var connection = await OpenConnectionAsync();
        var binary = new HashSet<string>(StringComparer.OrdinalIgnoreCase);

        await using (var types = new MySqlCommand(
            "SELECT COLUMN_NAME FROM information_schema.COLUMNS WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = @p0 AND DATA_TYPE IN ('binary', 'varbinary', 'tinyblob', 'blob', 'mediumblob', 'longblob')", connection))
        {
            types.Parameters.AddWithValue("@p0", table);
            await using var names = await types.ExecuteReaderAsync();

            while (await names.ReadAsync())
            {
                binary.Add(names.GetString(0));
            }
        }

        var select = string.Join(", ", columns.Select(x => binary.Contains(x) ? $"CONCAT('0x', HEX(`{x}`))" : $"CAST(`{x}` AS CHAR)"));
        await using var command = new MySqlCommand($"SET time_zone = '+00:00'; SELECT {select} FROM {table} WHERE id = @p0", connection);
        command.Parameters.AddWithValue("@p0", id);
        await using var reader = await command.ExecuteReaderAsync();

        if (!await reader.ReadAsync())
        {
            throw new InvalidOperationException($"No {table} row with id {id}.");
        }

        var row = new Dictionary<string, string?>(StringComparer.OrdinalIgnoreCase);

        for (var i = 0; i < columns.Length; i++)
        {
            row[columns[i]] = reader.IsDBNull(i) ? null : reader.GetString(i);
        }

        return row;
    }
}

  > 1. Commit: an account and its contacts, kept together
      before commit, other connections see 0 Woodgrove accounts
      after commit: 1 account, 2 contacts
  > 2. Roll back: your own check fails after the account was created
      after rollback: 0 Margie's Travel accounts
  > 3. A failed call aborts the transaction: ARX refuses to commit it
      caught ArxDatabaseException: ARX3001 UniqueViolation: Create on 'account' violates unique key 'account_account_number_key'.
      transaction.IsAborted = True
      caught ArxValidationException: ARX1015 TransactionAborted: An earlier call in this transaction failed at the database, so the transaction can only be rolled back: ARX doesn't commit part of a unit of work. It was rolled back instead of committed.
      stored: 0 Alpine Ski House accounts