only validate gpg_key#fingerprint "internally"

app/models/gpg_key.rb

@@ -3,17 +3,21 @@ class GpgKey < ActiveRecord::Base
 
   belongs_to :user
 
-  validates :fingerprint,
-    presence: true,
-    uniqueness: true
-
   validates :key,
     presence: true,
     uniqueness: true,
     format: {
-      with: /\A#{KEY_PREFIX}((?!#{KEY_PREFIX}).)+\Z/m
+      with: /\A#{KEY_PREFIX}((?!#{KEY_PREFIX}).)+\Z/m,
+      message: "is invalid. A valid public GPG key begins with '#{KEY_PREFIX}'"
     }
 
+  validates :fingerprint,
+    presence: true,
+    uniqueness: true,
+    # only validate when the `key` is valid, as we don't want the user to show
+    # the error about the fingerprint
+    unless: -> { errors.has_key?(:key) }
+
   before_validation :extract_fingerprint
 
   def key=(value)

spec/models/gpg_key_spec.rb

@@ -6,8 +6,6 @@ describe GpgKey do
   end
 
   describe "validation" do
-    it { is_expected.to validate_presence_of(:fingerprint) }
-
     it { is_expected.to validate_presence_of(:key) }
     it { is_expected.to validate_uniqueness_of(:key) }
     it { is_expected.to allow_value("-----BEGIN PGP PUBLIC KEY BLOCK-----\nkey").for(:key) }