Macro for defining a standalone constraint module.

Model-bound unique/2, unique_event_type/1, and remove_constraint/1,2 attributes on an event type (see Chronicle.Events.EventType) cover the common case: a constraint that belongs to exactly one event type's own module. use Chronicle.Events.Constraint is for everything else — a constraint spanning several event types under one name, or one you want to define apart from any single event type's module. It mirrors the C# client's IConstraint/IConstraintBuilder.

Defining a standalone constraint

defmodule MyApp.Constraints.UniqueEmail do
  use Chronicle.Events.Constraint

  unique name: "UniqueEmail" do
    on MyApp.Events.UserRegistered, [:email]
    on MyApp.Events.UserEmailChanged, [:new_email]
    removed_with MyApp.Events.UserRemoved
  end
end

A unique event type constraint, with a releasing event declared afterward:

defmodule MyApp.Constraints.OneOpenLoan do
  use Chronicle.Events.Constraint

  unique_event_type MyApp.Events.LoanCheckedOut, name: "OneOpenLoan"
  removed_with MyApp.Events.LoanReturned
end

Two unique_event_type/1,2 declarations sharing a name become one constraint — mutual exclusion: at most one event drawn from the named set is ever appended for the same event source:

defmodule MyApp.Constraints.PersonTerminalOutcome do
  use Chronicle.Events.Constraint

  unique_event_type MyApp.Events.PersonAliasedTo, name: "PersonTerminal"
  unique_event_type MyApp.Events.PersonErased, name: "PersonTerminal"
end

Registering

Standalone constraint modules are discovered automatically (like event types and read models, through :otp_app or loaded-module discovery) or registered explicitly:

{Chronicle.Client,
  connection_string: "chronicle://localhost:35000",
  event_store: "my-store",
  constraints: [MyApp.Constraints.UniqueEmail]}

unique/2 options

• :name — (required) the constraint name
• :mode — :per_event_source (default) or :per_value; see Chronicle.Events.EventType.unique/2
• :ignore_casing — compare values case-insensitively
• :message — a string, or a {Module, :function} callback invoked with the violation to compute one; see Chronicle.Events.ConstraintViolation.details/1
• :scope — see Chronicle.Events.EventType.unique/2
• :event_sequences — restrict the declaration to specific event sequences (default: all)

on/2

Declares one of the event types (and the properties on it) that participate in the enclosing unique/2 constraint. Call it more than once in the same block to constrain several event types under one name — the same event type cannot be added twice to one unique/2 block.

removed_with/1,2

Inside a unique/2 ... do ... end block, declares a removal event. With properties:, the removal carries the value to release in :per_value mode, in the same order as the constrained properties; omitted or empty releases every value for the event source. After a bare unique_event_type/1,2, releases that constraint — there is no properties:, since a unique event type constraint has no constrained value. Calling it before any unique_event_type/1,2 has been declared in the module is a compile-time error.

unique_event_type/1,2 options

• :name — (required) the constraint name
• :message — see above
• :scope — see above
• :event_sequences — see above