Double entry modeling

invariants.clj

(def billing-violations [negative-entries small-minimum-payment prepaid-mutual])

(def loss-violations [credit-loss pre-loss])

(def general-violations [minimum-payment-mutual])

(defn all-violations [balances]
  (concat billing-violations loss-violations general-violations))

(s/defn fixing-invariants :- [Entry] [initial-balances :- Balances, new-entries :- [Entry]]
    ;; Pre-checks that the DB starts in valid state    
    (when-let [violations (all-violations initial-balances)] (ex/invalid-input! {:reason :wrong-state-before-fixing-invariants
                                                                                 :balances   balances
                                                                                 :violations violations}))    
    (let [new-balances (apply-entries initial-balances new-entries)
          violations (apply-all all-violations new-balances)
          ;; if there are violations, try to fix them automatically
          [new-entries final-balances] (reduce (fn [[new-entries current-balance] violation]
                                                   (let [corrections  (case violation
                                                                         :negative-entry (fix-negative-entry current-balance)
                                                                         :prepaid-mutual (fix-prepaid-mutual current-balance)
                                                                         :minimum-payment-mutual (fix-minimum-payment-mutual current-balance)
                                                                         :credit-loss (fix-credit-loss current-balance)
                                                                         :pre-loss (fix-pre-loss current-balance)
                                                                         :small-minimum-payment (fix-small-minimum-payment current-balance))]
                                                      [(concat corrections new-entries) (apply-entries current-balance corrections)]))
                                              violations)]
        ;; If we weren't able to fix all of them, error!                                      
        (when-let [violations (not-empty (all-violations final-balances))]
          (ex/invalid-input! {:reason :invariant-violations-after-fixing-them :balances final-balances :violations violations}))
        new-entries))))
      

models.clj

(defschema Movement
  {:movement/id            UUID
   :movement/flow-id       String
   :movement/topic         TopicEnum
   :movement/owner-account Account
   :movement/produced-at   LocalDateTime
   :movement/consumed-at   LocalDateTime
   :movement/user          String})

(defschema Entry
  {:entry/id             UUID
   :entry/amount         t-money/PositiveAmount
   :entry/debit-account  BookAccountType ; all of our book accounts
   :entry/credit-account BookAccountType
   :entry/post-date      LocalDate
   :entry/movement       Movement})

rules.clj

(def new-purchase [{:entry/debit-account  :book-account-type.asset/unsettled
                    :entry/credit-account :book-account-type.liability/unsettled-counterparty
                    :entry/amount         #'transaction-amount ; function that extracts this amount from the event payload
                    :entry/post-date      #'produced-date}
                   {:entry/debit-account  :book-account-type.liability/current-limit-counterparty
                    :entry/credit-account :book-account-type.asset/current-limit
                    :entry/amount         #'transaction-amount
                    :entry/post-date      #'produced-date}])
                        

(def settle-purchase  [{:entry/debit-account  :book-account-type.asset/settled-brazil
                        :entry/credit-account :book-account-type.asset/installments-merchant
                        :entry/amount         #'amount
                        :entry/post-date      #'post-date}
                       {:entry/debit-account  :book-account-type.liability/payable-installments
                        :entry/credit-account :book-account-type.liability/payable-brazil
                        :entry/amount         #'amount
                        :entry/post-date      #'post-date}])