Raise when fast_distinct_count is used on primary key

.rubocop.yml

@@ -17,6 +17,9 @@ Style/SymbolArray:
 Style/WordArray:
   EnforcedStyle: brackets
 
+Style/IfUnlessModifier:
+  Enabled: false
+
 Layout/IndentationConsistency:
   EnforcedStyle: indented_internal_methods
 

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## master (unreleased)
 
+- Raise when `fast_distinct_count` is used on primary key
+
 ## 0.2.0 (2023-07-24)
 
 - Support for quickly getting an exact number of distinct values in a column

lib/fast_count/extensions.rb

@@ -31,6 +31,10 @@ def fast_count(threshold: FastCount.threshold)
       #   User.fast_distinct_count(column: :company_id)
       #
       def fast_distinct_count(column:)
+        if column.to_s == primary_key
+          raise "Use `#fast_count` when counting primary keys."
+        end
+
         adapter = Adapters.for_connection(connection)
         adapter.fast_distinct_count(table_name, column)
       end

test/fast_count_test.rb

@@ -121,6 +121,13 @@ def test_fast_distinct_count
     @connection.remove_index(:users, :company_id)
   end
 
+  def test_fast_distinct_count_on_primary_key
+    error = assert_raises(RuntimeError) do
+      User.fast_distinct_count(column: :id)
+    end
+    assert_equal "Use `#fast_count` when counting primary keys.", error.message
+  end
+
   def test_fast_distinct_count_counts_nulls
     @connection.add_index(:users, :company_id)
     @connection.execute(<<~SQL)