Convention-Based Mapping with FromEvent

The FromEvent attribute enables convention-based automatic property mapping between events and read models, similar to the .AutoMap() functionality in regular projections. This attribute automatically maps properties with matching names, eliminating the need for explicit property-level attributes in many cases.

How FromEvent Works

FromEvent performs automatic property mapping using these rules:

Name matching: Properties with identical names (case-sensitive) between event and read model are automatically mapped
Type compatibility: Property types must be compatible for assignment
Recursive mapping: Nested objects are automatically mapped recursively if their property names match
Collection handling: Arrays and collections are handled automatically
Selective mapping: Only properties that exist on both event and read model are mapped (missing properties are silently ignored)

This approach is equivalent to using .AutoMap() in regular projection classes but achieved through attributes.

Basic Convention Mapping

Apply FromEvent at the class level to enable automatic property matching:

using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;

[FromEvent<UserRegistered>]
public record User(
    [Key]
    Guid Id,

    string Name,        // Automatically mapped from UserRegistered.Name
    string Email,       // Automatically mapped from UserRegistered.Email
    DateTimeOffset RegisteredAt);  // Automatically mapped from UserRegistered.RegisteredAt

This is equivalent to:

public record User(
    [Key]
    Guid Id,

    [SetFrom<UserRegistered>(nameof(UserRegistered.Name))]
    string Name,

    [SetFrom<UserRegistered>(nameof(UserRegistered.Email))]
    string Email,

    [SetFrom<UserRegistered>(nameof(UserRegistered.RegisteredAt))]
    DateTimeOffset RegisteredAt);

Multiple Events

You can use multiple FromEvent attributes for different events:

[FromEvent<UserRegistered>]
[FromEvent<UserProfileUpdated>]
public record UserProfile(
    [Key]
    Guid Id,

    string Name,     // From UserRegistered and UserProfileUpdated
    string Email,    // From UserRegistered and UserProfileUpdated
    string Phone);   // From UserProfileUpdated only

Properties are matched by name. If an event doesn't have a matching property, that property is skipped for that event.

Custom Key Specification

By default, FromEvent uses the event source identifier to identify which read model instance to update. You can specify a different property from the event to use as the key:

[FromEvent<UserRegistered>(key: nameof(UserRegistered.UserId))]
public record User(
    [Key]
    Guid Id,

    string Name,
    string Email);

This is equivalent to using .UsingKey() in declarative projections:

public class UserProjection : IProjectionFor<User>
{
    public void Define(IProjectionBuilderFor<User> builder) => builder
        .AutoMap()
        .From<UserRegistered>(_ => _
            .UsingKey(e => e.UserId));
}

When to Use Custom Keys

Use the key parameter when:

Event source ID doesn't match read model key: The event's natural identifier differs from the event source ID
Multiple instances per source: A single event source creates multiple read model instances
Cross-aggregate projections: Events from one aggregate update read models keyed by a different aggregate

Example with Custom Key

[EventType]
public record OrderLineItemAdded(
    Guid OrderId,      // Key for the Order read model
    Guid LineItemId,   // Key for individual line items
    string ProductName,
    int Quantity,
    decimal Price);

// Order projection using OrderId as key
[FromEvent<OrderLineItemAdded>(key: nameof(OrderLineItemAdded.OrderId))]
public record Order(
    [Key]
    Guid Id,

    // Properties auto-mapped from OrderLineItemAdded
    decimal TotalAmount);

Key Validation

The key property must exist on the event type. If you specify a non-existent property, you'll get a compile-time error:

// ❌ This will throw InvalidPropertyForType exception
[FromEvent<UserRegistered>(key: "NonExistentProperty")]
public record User([Key] Guid Id, string Name);

Mixing Convention and Explicit Mapping

You can combine FromEvent with explicit property attributes:

[FromEvent<AccountOpened>]
public record Account(
    [Key]
    Guid Id,

    string Name,  // Convention-based from AccountOpened.Name

    [SetFrom<AccountOpened>(nameof(AccountOpened.InitialBalance))]
    [AddFrom<DepositMade>(nameof(DepositMade.Amount))]
    [SubtractFrom<WithdrawalMade>(nameof(WithdrawalMade.Amount))]
    decimal Balance);  // Explicit mapping for complex operations

In this example:

Name is mapped automatically by convention
Balance uses explicit attributes for multiple operations

Relationship to Regular Projections AutoMap

FromEvent in model-bound projections provides the same automatic mapping functionality as .AutoMap() in regular projection classes. These approaches are equivalent:

Model-bound projection with FromEvent:

[FromEvent<UserRegistered>]
[FromEvent<UserProfileUpdated>]
public record User(
    [Key] Guid Id,
    string Name,
    string Email,
    string Phone);

Regular projection with AutoMap:

public class UserProjection : IProjectionFor<User>
{
    public void Define(IProjectionBuilderFor<User> builder) => builder
        .AutoMap()
        .From<UserRegistered>()
        .From<UserProfileUpdated>();
}

Both approaches automatically map properties with matching names and types.

Nested Object Mapping

FromEvent automatically handles nested object mapping when property names and structures match:

// Events with nested structures
[EventType]
public record CustomerRegistered(
    string FirstName,
    string LastName,
    string Email,
    Address BillingAddress,
    Address ShippingAddress);

[EventType]
public record CustomerAddressUpdated(
    Address BillingAddress,
    Address ShippingAddress);

public record Address(
    string Street,
    string City,
    string State,
    string ZipCode,
    string Country);

// Read model with automatic nested mapping
[FromEvent<CustomerRegistered>]
[FromEvent<CustomerAddressUpdated>]
public record Customer(
    [Key] Guid Id,

    // Simple properties mapped automatically
    string FirstName,
    string LastName,
    string Email,

    // Nested objects mapped recursively
    Address BillingAddress,
    Address ShippingAddress);

Collection Mapping

Arrays and collections are automatically mapped when the element types and property names match:

[EventType]
public record OrderCreated(
    string CustomerEmail,
    LineItem[] Items,
    string[] Tags);

public record LineItem(
    string ProductName,
    decimal UnitPrice,
    int Quantity);

[FromEvent<OrderCreated>]
public record Order(
    [Key] Guid Id,

    string CustomerEmail,  // Automatically mapped
    LineItem[] Items,      // Array mapped with nested objects
    string[] Tags);        // Simple array mapped

Complete Example

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;

// Events with matching property names
[EventType]
public record EmployeeHired(
    string FirstName,
    string LastName,
    string Email,
    string Department,
    DateTimeOffset HiredAt);

[EventType]
public record EmployeeDepartmentChanged(
    string Department,
    DateTimeOffset ChangedAt);

[EventType]
public record SalaryAdjusted(decimal NewSalary);

// Read Model using convention-based mapping
[FromEvent<EmployeeHired>]
[FromEvent<EmployeeDepartmentChanged>]
public record Employee(
    [Key]
    Guid Id,

    // These properties are automatically mapped by name
    string FirstName,
    string LastName,
    string Email,
    string Department,

    // This needs explicit mapping because it comes from a different property name
    [SetFrom<SalaryAdjusted>(nameof(SalaryAdjusted.NewSalary))]
    decimal Salary,

    // Track department changes
    [Count<EmployeeDepartmentChanged>]
    int DepartmentChangeCount);

Automatic Mapping Benefits

FromEvent provides the same benefits as AutoMap:

Reduces boilerplate: Eliminates the need for individual SetFrom attributes on each property
Convention over configuration: Property names drive the mapping automatically
Type safety: Compile-time checking ensures compatible types
Nested support: Automatically handles complex object hierarchies
Collection handling: Arrays and lists are mapped recursively
Maintainable: Renaming properties updates mappings automatically

When to Use Convention-Based Mapping

Use FromEvent when:

Property names match exactly between events and read models
You're mapping multiple properties from the same event
You want to reduce boilerplate code
The mapping is straightforward (simple Set operations)
You have consistent naming conventions across events and read models
You need to map nested objects or collections with matching structures

Use explicit attributes when:

Property names don't match between event and read model
You need complex operations (Add, Subtract, Increment, Count, etc.)
You're mapping from multiple events with different operations
You want explicit control over each property mapping
Properties require transformations or calculations
You're mapping from different event property names

Property Matching Rules

Name must match exactly - Property names are case-sensitive
Type compatibility - Properties must be compatible types (automatic conversions may apply)
Event must have property - If the event doesn't have a matching property, it's skipped (no error)
Read model must be writable - Properties must be settable (which they are in records)

Benefits

Convention-based mapping with FromEvent:

Reduces verbosity - Less code to write and maintain
Improves readability - Clear intent when most properties map directly
Easier refactoring - Rename properties in both places without updating attributes
Best of both worlds - Combine with explicit attributes where needed

Performance Considerations

Like .AutoMap() in regular projections, FromEvent performs property matching at projection definition time, not during event processing. There is no runtime performance penalty compared to explicit SetFrom attributes - both compile to the same internal representation.

Advanced Scenarios

Type Compatibility

FromEvent handles common type conversions automatically:

[EventType]
public record ProductPriceChanged(double NewPrice);  // double in event

[FromEvent<ProductPriceChanged>]
public record Product(
    [Key] Guid Id,
    decimal NewPrice);  // decimal in read model - automatically converted

Partial Property Mapping

Events don't need to have all read model properties. Missing properties are ignored:

[EventType]
public record UserRegistered(string Email);  // Only has Email

[EventType]
public record UserProfileCompleted(string FirstName, string LastName, string Phone);

[FromEvent<UserRegistered>]      // Maps: Email
[FromEvent<UserProfileCompleted>] // Maps: FirstName, LastName, Phone
public record User(
    [Key] Guid Id,
    string Email,      // From UserRegistered
    string FirstName,  // From UserProfileCompleted
    string LastName,   // From UserProfileCompleted
    string Phone);     // From UserProfileCompleted

Combining with Other Projection Features

FromEvent works seamlessly with other model-bound projection features:

[FromEvent<AccountOpened>]           // Auto-maps: AccountNumber, CustomerName
[FromEvent<AccountDetailsUpdated>]   // Auto-maps: CustomerName, ContactEmail
public record Account(
    [Key] Guid Id,

    // Auto-mapped properties
    string AccountNumber,
    string CustomerName,
    string ContactEmail,

    // Explicit mappings for complex scenarios
    [SetFrom<AccountOpened>(nameof(AccountOpened.InitialDeposit))]
    [AddFrom<DepositMade>(nameof(DepositMade.Amount))]
    [SubtractFrom<WithdrawalMade>(nameof(WithdrawalMade.Amount))]
    decimal Balance,

    // Metadata tracking
    [FromEvery(contextProperty: "Occurred")]
    DateTimeOffset LastModified);

Best Practices

Use consistent naming conventions between events and read models to maximize automatic mapping effectiveness
Start with FromEvent and add explicit attributes only where needed for complex scenarios
Document mixed approaches with comments when combining automatic and explicit mappings
Consider maintainability - Sometimes explicit attributes provide better clarity even if more verbose
Leverage nested mapping - Design nested types with consistent property names for recursive automatic mapping
Group related events - Use multiple FromEvent attributes for events that share similar property structures

Summary

The FromEvent attribute provides powerful automatic property mapping capabilities that mirror the .AutoMap() functionality in regular projections. By following consistent naming conventions between events and read models, you can significantly reduce boilerplate code while maintaining type safety and performance. Use FromEvent as your starting point for model-bound projections, and add explicit attributes only when you need custom mapping logic or complex operations.

This automatic mapping approach makes model-bound projections both concise and maintainable, allowing you to focus on your domain logic rather than repetitive property mapping code.

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