From Event

The from directive defines a rule that triggers when a specific event occurs. This is the primary way to populate and update read models from events.

Basic Syntax

from {EventType}
  {mappings and operations}

Simple Example

projection User => UserReadModel
  from UserRegistered
    Name = name
    Email = email
    IsActive = true

When a UserRegistered event occurs, it creates or updates a UserReadModel instance with the specified properties.

Multiple Events (Compact Syntax)

You can define multiple events on the same line separated by commas when they share the same mappings and configuration:

projection TransportRoute => TransportRouteReadModel
  automap
  from HubRouteAdded key id, WarehouseRouteAdded key id

This is equivalent to:

projection TransportRoute => TransportRouteReadModel
  automap
  from HubRouteAdded key id
  from WarehouseRouteAdded key id

Each event can have its own inline key specification:

from EventA key idA, EventB key idB, EventC
  automap
  Property = value

With Keys

Specify which property identifies the projection instance:

from UserAssignedToGroup key userId
  GroupId = $eventContext.eventSourceId
  AssignedAt = $eventContext.occurred

See Keys for more details on key handling.

With AutoMap

Automatically map matching properties:

from UserRegistered automap
  IsActive = true

AutoMap copies properties with matching names from the event to the read model, then applies any explicit mappings.

See Auto-Map for more details.

Multiple From Blocks

A projection can have multiple from blocks for different events:

projection User => UserReadModel
  from UserRegistered
    Name = name
    Email = email
    CreatedAt = $eventContext.occurred

  from UserEmailChanged
    Email = email
    UpdatedAt = $eventContext.occurred

  from UserDeactivated
    IsActive = false
    DeactivatedAt = $eventContext.occurred

With Parent Key

When used within Children blocks, you can specify the parent relationship:

children members id userId
  from UserAddedToGroup key userId
    parent groupId
    Role = role

Operations

Within a from block, you can use:

• Property assignments: Name = value
• Counters: increment, decrement, count
• Arithmetic: add, subtract
• AutoMap: Automatically map matching properties
• Keys: Specify instance identification
• Composite Keys: Multi-property keys

Event Context

Access event metadata using $eventContext:

from UserLoggedIn
  LastLogin = $eventContext.occurred
  LastSequenceNumber = $eventContext.sequenceNumber
  count LoginCount

See Event Context for available properties.

Nested Indentation

All mappings and operations within a from block must be indented:

from UserCreated
  Name = name          # Indented
  Email = email        # Indented
  IsActive = true      # Indented

Examples

Simple Property Mapping

from OrderPlaced
  CustomerId = customerId
  Total = total
  Status = "Pending"

Using Templates

from PersonRegistered
  FullName = `${firstName} ${lastName}`
  Email = email

Counter Operations

from PageViewed
  count ViewCount
  LastViewedAt = $eventContext.occurred

Arithmetic Operations

from PaymentReceived
  add Balance by amount
  LastPaymentDate = $eventContext.occurred

Complex Example

from OrderPlaced key orderId
  CustomerId = customerId
  OrderNumber = orderNumber
  Total = total
  Status = "Pending"
  PlacedAt = $eventContext.occurred
  increment TotalOrders