As of Active Record 7.1.0.beta1, the presence validation added by a

belongs_to association can now have an associated lambda. This lambda is
just to check whether the column has changed to save redundant
validations, so we can ignore it.

Author: RaymondFFT

lib/rails_admin/adapters/active_record.rb

@@ -135,6 +135,10 @@ def parse_id(id)
         end
       end
 
+      def belongs_to_required_by_default
+        model.belongs_to_required_by_default
+      end
+
     private
 
       def primary_key_scope(scope, id)

lib/rails_admin/adapters/mongoid.rb

@@ -112,6 +112,10 @@ def adapter_supports_joins?
         false
       end
 
+      def belongs_to_required_by_default
+        ::Mongoid.belongs_to_required_by_default
+      end
+
     private
 
       def build_statement(column, type, value, operator)

lib/rails_admin/config/fields/base.rb

@@ -216,11 +216,18 @@ def filter_options
             end
 
           (@required ||= {})[context] ||= !!([name] + children_fields).uniq.detect do |column_name|
-            abstract_model.model.validators_on(column_name).detect do |v|
+            model = abstract_model.model
+            model.validators_on(column_name).detect do |v|
               !(v.options[:allow_nil] || v.options[:allow_blank]) &&
                 %i[presence numericality attachment_presence].include?(v.kind) &&
                 (v.options[:on] == context || v.options[:on].blank?) &&
                 (v.options[:if].blank? && v.options[:unless].blank?)
+            end || model.reflect_on_all_associations(:belongs_to).detect do |a|
+              next unless a.name == column_name
+
+              required = a.options[:required] if a.options.key?(:required)
+              required = !a.options[:optional] if a.options.key?(:optional) && required.nil?
+              required.nil? ? abstract_model.belongs_to_required_by_default : required
             end
           end
         end

spec/rails_admin/config/fields/base_spec.rb

@@ -86,9 +86,15 @@ class RelTest < Tableless
           column :league_id, :integer
           column :division_id, :integer, nil, false
           column :player_id, :integer
+          column :team_id, :integer
+          column :draft_id, :integer
+          column :image_id, :integer
           belongs_to :league, optional: true
           belongs_to :division, optional: true
           belongs_to :player, optional: true
+          belongs_to :team, optional: false
+          belongs_to :draft, required: true
+          belongs_to :image, required: false
           validates_numericality_of(:player_id, only_integer: true)
         end
         @fields = RailsAdmin.config(RelTest).create.fields
@@ -111,6 +117,24 @@ class RelTest < Tableless
           expect(@fields.detect { |f| f.name == :player }.required?).to be_truthy
         end
       end
+
+      describe 'for belongs_to association with optional: false' do
+        it 'is required' do
+          expect(@fields.detect { |f| f.name == :team }.required?).to be_truthy
+        end
+      end
+
+      describe 'for belongs_to association with required: true' do
+        it 'is required' do
+          expect(@fields.detect { |f| f.name == :draft }.required?).to be_truthy
+        end
+      end
+
+      describe 'for belongs_to association with required: false' do
+        it 'is optional' do
+          expect(@fields.detect { |f| f.name == :image }.required?).to be_falsey
+        end
+      end
     end
   end