GridWeapon decides if it is mainhand or not

app/controllers/api/v1/grid_weapons_controller.rb

@@ -167,6 +167,7 @@ module Api
           output = render_conflict_view([conflict_weapon], incoming_weapon, weapon_params[:position])
           render json: output
         else
+          ap "Moving weapon to new position..."
           # Move the original grid weapon to the new position
           # to preserve keys and other modifications
           old_position = conflict_weapon.position

app/models/grid_weapon.rb

@@ -13,6 +13,8 @@ class GridWeapon < ApplicationRecord
   validate :compatible_with_position, on: :create
   validate :no_conflicts, on: :create
 
+  before_save :is_mainhand
+
   ##### Amoeba configuration
   amoeba do
     nullify :ax_modifier1
@@ -71,4 +73,13 @@ class GridWeapon < ApplicationRecord
     # Check if the grid weapon conflicts with any of the other grid weapons in the party
     errors.add(:series, 'must not conflict with existing weapons') unless conflicts(party).nil?
   end
+
+  # Checks if the weapon should be a mainhand before saving the model
+  def is_mainhand
+    if self.position == -1
+      self.mainhand = true
+    else
+      self.mainhand = false
+    end
+  end
 end