Account balance anchors

app/models/valuation.rb

@@ -1,3 +1,29 @@
 class Valuation < ApplicationRecord
   include Entryable
+
+  enum :kind, {
+    recon: "recon", # A balance reconciliation that sets the Account balance from this point forward (often defined by user)
+    snapshot: "snapshot", # An "event-sourcing snapshot", which is purely for performance so less history is required to derive the balance
+    opening_anchor: "opening_anchor", # Each account has a single opening anchor, which defines the opening balance on the account
+    current_anchor: "current_anchor" # Each account has a single current anchor, which defines the current balance on the account
+  }, validate: true
+
+  # Each account can have at most 1 opening anchor and 1 current anchor. All valuations between these anchors should
+  # be either "recon" or "snapshot". This ensures we can reliably construct the account balance history solely from Entries.
+  validate :unique_anchor_per_account, if: -> { opening_anchor? || current_anchor? }
+
+  private
+    def unique_anchor_per_account
+      return unless entry&.account
+
+      existing_anchor = entry.account.valuations
+                             .joins(:entry)
+                             .where(kind: kind)
+                             .where.not(id: id)
+                             .exists?
+
+      if existing_anchor
+        errors.add(:kind, "#{kind.humanize} already exists for this account")
+      end
+    end
 end