Message Templates

<!-- Variable substitution -->
Hello {{ admin.name }},

<!-- Conditional content -->
{{ if admin.is_protected }}
  You are the super admin for {{ tenant.name }}.
{{ end }}

<!-- Loops -->
{{ for role in admin.roles }}
  - {{ role.name }}
{{ end }}

<!-- Filters (pipes) -->
Created: {{ created_at | date.to_string "%B %d, %Y" }}
Amount: {{ amount | math.format "0.00" }}

public class MessageTemplate : AuditableEntity<Guid>
{
    [Required] [MaxLength(100)]
    public string Key { get; set; } = null!;       // Unique key, e.g., "welcome_admin"
    
    public MessageChannel Channel { get; set; }    // Email, SMS, Push
    
    [MaxLength(500)]
    public string? Subject { get; set; }           // Subject with variables
    
    [Required] [MaxLength(50000)]
    public string Body { get; set; } = null!;      // HTML/Text body with Scriban template syntax
    
    [Required] [MaxLength(10)]
    public string Language { get; set; } = "en";   // "en" or "ar"
    
    public new bool IsActive { get; set; } = true;
    public Guid? TenantId { get; set; }            // Null = system-wide, set = tenant override
    
    [MaxLength(500)]
    public string? Description { get; set; }
    
    [MaxLength(4000)]
    public string? PlaceholderSchema { get; set; } // JSON array of placehoders (key, type, required)
    
    [MaxLength(2000)]
    public string? DesignVariables { get; set; }   // Theme-able CSS variables in JSON format
    
    public int Version { get; set; } = 1;          // Auto-incremented version number
    
    [MaxLength(50)]
    public string? Category { get; set; }          // e.g. Transactional, Marketing
    
    [MaxLength(500)]
    public string? Tags { get; set; }
}

public class TemplateRenderer : ITemplateRenderer
{
    private static string RenderWithScriban(string templateText, Dictionary<string, object> data)
    {
        var parsedTemplate = Template.Parse(templateText);

        if (parsedTemplate.HasErrors)
        {
            // Syntax error fallback: Safe literal double curly braces replacement
            var result = templateText;
            foreach (var kvp in data)
            {
                result = result.Replace($"{{{{{kvp.Key}}}}}", kvp.Value?.ToString() ?? "");
            }
            return result;
        }

        var scriptObject = new ScriptObject();
        foreach (var kvp in data)
        {
            scriptObject[kvp.Key] = kvp.Value;
        }

        var context = new TemplateContext();
        context.PushGlobal(scriptObject);
        return parsedTemplate.Render(context);
    }
}

// Validation rule enforcing correct JSON syntax on placeholder schema & design variables
RuleFor(x => x.PlaceholderSchema)
    .MaximumLength(4000)
    .Must(BeValidJson)
    .When(x => !string.IsNullOrWhiteSpace(x.PlaceholderSchema))
    .WithMessage("PlaceholderSchema must be valid JSON.");

private static bool BeValidJson(string? json)
{
    if (string.IsNullOrWhiteSpace(json)) return true;
    try
    {
        System.Text.Json.JsonDocument.Parse(json);
        return true;
    }
    catch { return false; }
}

// Validate Scriban syntax and increment template version on change
var parsed = Template.Parse(request.Body);
if (parsed.HasErrors)
    return Error.Validation("Template body has syntax errors.");

template.Subject = request.Subject;
template.Body = request.Body;
template.PlaceholderSchema = request.PlaceholderSchema;
template.Version += 1; // Auto-increment version

await _unitOfWork.SaveChangesAsync(cancellationToken);

// POST /api/v1/message-templates/preview
{
  "subject": "Welcome {{ admin.name }}",
  "body": "<p>Hello {{ admin.name }}, welcome to {{ tenant.name }}!</p>",
  "sampleData": {
    "admin": { "name": "John Doe", "email": "john@example.com" },
    "tenant": { "name": "ACME Corp" }
  }
}

// Response:
// {
//   "subject": "Welcome John Doe",
//   "body": "<p>Hello John Doe, welcome to ACME Corp!</p>"
// }