Class: StandardLedger::Projection

Inherits:
Object
  • Object
show all
Defined in:
lib/standard_ledger/projection.rb

Overview

Base class for projector classes registered via ‘projects_onto :target, via: ProjectorClass`. Subclasses implement `apply` (and optionally `rebuild`) to mutate the target based on a new entry.

For projections expressible as block DSL (counter increments, simple delta updates), prefer the block form on ‘Projector#projects_onto` instead — extracting a class is for non-trivial projectors.

Examples:

class Orders::FulfillableProjector < StandardLedger::Projection
  # Called inside the async job, with target locked.
  def apply(order, _entry)
    order.update!(
      fulfillable_balance: order.fulfillment_records.group(:key).sum(:amount),
      fulfillable_status:  order.fulfillment_records.group(:key).sum(:amount).values.all?(&:zero?) ? :fulfilled : :pending
    )
  end

  # Called by Projection.rebuild! to recompute from the full log.
  def rebuild(order)
    apply(order, nil)
  end
end

Instance Method Summary collapse

Instance Method Details

#apply(_target, _entry) ⇒ Object

Apply a single entry’s effect to the target. Called inside the transactional or async boundary of the chosen mode.

Raises:

  • (NotImplementedError) 


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# File 'lib/standard_ledger/projection.rb', line 29

def apply(_target, _entry)
  raise NotImplementedError, "#{self.class}#apply must be implemented"
end

#rebuild(_target) ⇒ Object

Recompute the target’s projection from the full entry log. Called by ‘StandardLedger.rebuild!`. Projectors that cannot be rebuilt (e.g. delta-only `increment_counter` flavored ones) should raise `StandardLedger::NotRebuildable`.

Raises:



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# File 'lib/standard_ledger/projection.rb', line 37

def rebuild(_target)
  raise NotRebuildable, "#{self.class}#rebuild not implemented; this projector cannot be rebuilt from the entry log"
end