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
  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

AutoMap is on by default, so matching properties are copied without asking for it:

from UserRegistered
  IsActive = true

AutoMap copies properties with matching names from the event to the read model, then applies any explicit mappings. AutoMap cannot be turned off inside a from block (PLAY0075); use no automap at the projection, children, every/all or join with level.

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

One use of an event per projection level

Chronicle allows one use of an event type per projection level. Ordinary compilation reports PLAY0801 at the second and each later reference, even if the keys are different or identical. The check compares authored event names ordinally; it does not resolve qualifications or generations. One set covers from (including comma-separated events), remove with, remove via join and clear with. Joins have a separate set: an event can occur once in from and once in a join. Each children, nested and variant block starts fresh sets; different projections are independent. Variants still do not bind to the executable model.

One event cannot update two rows of the same level by appearing in two from blocks. Combine mappings that update the same row into one block. Model a genuinely second domain fact as its own event, produced by the command or a reaction when that fact actually occurs; do not invent events solely for a read-side update. Two keys do not turn one fact into two facts.

With Parent Key

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

children members identified by 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