Account:: namespace simplifications and cleanup (#2110)

* Flatten Holding model

* Flatten balance model

* Entries domain renames

* Fix valuations reference

* Fix trades stream

* Fix brakeman warnings

* Fix tests

* Replace existing entryable type references in DB

app/models/transaction/transferable.rb

@@ -0,0 +1,40 @@
+module Transaction::Transferable
+  extend ActiveSupport::Concern
+
+  included do
+    has_one :transfer_as_inflow, class_name: "Transfer", foreign_key: "inflow_transaction_id", dependent: :destroy
+    has_one :transfer_as_outflow, class_name: "Transfer", foreign_key: "outflow_transaction_id", dependent: :destroy
+
+    # We keep track of rejected transfers to avoid auto-matching them again
+    has_one :rejected_transfer_as_inflow, class_name: "RejectedTransfer", foreign_key: "inflow_transaction_id", dependent: :destroy
+    has_one :rejected_transfer_as_outflow, class_name: "RejectedTransfer", foreign_key: "outflow_transaction_id", dependent: :destroy
+  end
+
+  def transfer
+    transfer_as_inflow || transfer_as_outflow
+  end
+
+  def transfer?
+    transfer.present?
+  end
+
+  def transfer_match_candidates
+    candidates_scope = if self.entry.amount.negative?
+      family_matches_scope.where("inflow_candidates.entryable_id = ?", self.id)
+    else
+      family_matches_scope.where("outflow_candidates.entryable_id = ?", self.id)
+    end
+
+    candidates_scope.map do |match|
+      Transfer.new(
+        inflow_transaction_id: match.inflow_transaction_id,
+        outflow_transaction_id: match.outflow_transaction_id,
+      )
+    end
+  end
+
+  private
+    def family_matches_scope
+      self.entry.account.family.transfer_match_candidates
+    end
+end