add Member#some_roles_deletable? instead of changing deletable?

app/contracts/members/delete_contract.rb

@@ -35,7 +35,7 @@ class DeleteContract < ::DeleteContract
     private
 
     def member_is_deletable
-      errors.add(:base, :not_deletable) unless model.deletable?
+      errors.add(:base, :not_deletable) unless model.some_roles_deletable?
     end
   end
 end

app/contracts/work_package_members/delete_contract.rb

@@ -35,7 +35,7 @@ class DeleteContract < ::DeleteContract
     private
 
     def member_is_deletable
-      errors.add(:base, :not_deletable) unless model.deletable?
+      errors.add(:base, :not_deletable) unless model.some_roles_deletable?
     end
   end
 end

app/models/member.rb

@@ -77,6 +77,10 @@ def <=>(other)
   end
 
   def deletable?
+    member_roles.none?(&:inherited_from?)
+  end
+
+  def some_roles_deletable?
     !member_roles.all?(&:inherited_from?)
   end
 

spec/contracts/members/delete_contract_spec.rb

@@ -60,7 +60,7 @@
 
   context 'when member is not deletable' do
     before do
-      allow(member).to receive(:deletable?).and_return(false)
+      allow(member).to receive(:some_roles_deletable?).and_return(false)
     end
 
     context 'for admin' do

spec/contracts/work_package_members/delete_contract_spec.rb

@@ -67,7 +67,7 @@
 
   context 'when member is not deletable' do
     before do
-      allow(member).to receive(:deletable?).and_return(false)
+      allow(member).to receive(:some_roles_deletable?).and_return(false)
     end
 
     context 'for admin' do

spec/models/member_spec.rb

@@ -55,17 +55,39 @@
   end
 
   describe '#deletable?' do
-    it 'is deletable, when no roles are inherited' do
+    it 'is true, when no roles are inherited' do
       member.member_roles.first.inherited_from = nil
       expect(member).to be_deletable
     end
 
-    it 'is not deletable, when roles are inherited' do
+    it 'is false, when roles are inherited' do
       member.member_roles.first.inherited_from = 1
       expect(member).not_to be_deletable
     end
   end
 
+  describe '#some_roles_deletable?' do
+    before do
+      member.roles << project_role
+    end
+
+    it 'is true, when no roles are inherited' do
+      member.member_roles.first.inherited_from = nil
+      expect(member).to be_some_roles_deletable
+    end
+
+    it 'is true, when not all roles are inherited' do
+      member.member_roles.first.inherited_from = 1
+      expect(member).to be_some_roles_deletable
+    end
+
+    it 'is false, when all roles are inherited' do
+      member.member_roles.first.inherited_from = 1
+      member.member_roles.second.inherited_from = 1
+      expect(member).not_to be_some_roles_deletable
+    end
+  end
+
   describe '#deletable_role?' do
     it 'can delete directly assigned roles, but not if role is inherited through a group membership' do
       # user has the global_role by directly being assigned