diff --git a/test/test_orm_id_lookup_index.rb b/test/test_orm_id_lookup_index.rb
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+++ b/test/test_orm_id_lookup_index.rb
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+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+
+# `find_by(id: x)`/`where(id: x).first` go through Relation#rows exactly like
+# any other equality lookup, which used to mean Store#lookup_by_index had to
+# find a registered secondary index covering the 'id' column - but no
+# framework code ever calls `add_index(table, :id)` (id is the primary key,
+# already a Hash key on definition[:rows]), so that search always failed and
+# silently fell back to a full table scan (Store#all). This was real, live
+# behavior, not a theoretical gap: Prism::Authenticatable#current_user calls
+# `authenticatable_model.find_by(id: id)` on every single authenticated
+# request (lib/prism/authenticatable.rb), and examples/blog's
+# ApplicationController#comment_post_label/#comment_post_linkable? call
+# `Post.with_deleted.find_by(id: comment.post_id)` once per rendered comment
+# row - both were full-scanning their table on every hit before this fix
+# (discovered via examples/blog's new AccountController#moderation page,
+# which renders several such rows per response and made the N full scans
+# impossible to miss in the sql.prism log). #lookup_by_index_in already had
+# this exact same "id is always indexed for free" special case for IN-lists
+# (see its own comment) - this closes the matching gap for a plain scalar
+# equality.
+class OrmIdLookupIndexTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-id-lookup-index-store')
+    @store = CountingIdIndexStore.new(File.join(@dir, 'test.pstore'))
+    @store.create_table(:widgets, %w[id sku])
+    @a = @store.insert(:widgets, sku: 'AAA')
+    @b = @store.insert(:widgets, sku: 'BBB')
+
+    widget_class = Class.new(Prism::Orm::Model) { table_name :widgets }
+    widget_class.store = @store
+    Object.const_set(:IdLookupWidget, widget_class)
+  end
+
+  def teardown
+    FileUtils.remove_entry(@dir)
+    Object.send(:remove_const, :IdLookupWidget) if Object.const_defined?(:IdLookupWidget)
+  end
+
+  def test_lookup_by_index_answers_a_bare_id_equality_without_any_registered_index
+    matches = @store.lookup_by_index(:widgets, { 'id' => @a })
+    refute_nil matches, 'a plain id equality must never report "no index" (nil means fall back to a full scan)'
+    assert_equal ['AAA'], matches.map { |row| row['sku'] }
+  end
+
+  def test_lookup_by_index_on_a_missing_id_is_an_empty_hit_not_a_scan_signal
+    assert_equal [], @store.lookup_by_index(:widgets, { 'id' => @a + 999 })
+  end
+
+  def test_extra_equality_keys_alongside_id_are_still_resolved_by_id_alone
+    # No index exists on `sku`, and the wrong sku is paired with the right
+    # id - the id lookup still returns the row (Relation#rows re-checks the
+    # full condition afterward and would reject it there, not here).
+    matches = @store.lookup_by_index(:widgets, { 'id' => @a, 'sku' => 'WRONG' })
+    assert_equal ['AAA'], matches.map { |row| row['sku'] }
+  end
+
+  def test_model_find_by_id_never_scans
+    @store.reset_counts!
+    widget = IdLookupWidget.find_by(id: @a)
+    assert_equal 'AAA', widget.sku
+    assert_equal 0, @store.scan_count
+    assert_equal 1, @store.id_index_hits
+  end
+
+  def test_where_id_first_never_scans_even_with_an_unrelated_condition_chained_on
+    @store.reset_counts!
+    widget = IdLookupWidget.where(id: @a).where(sku: 'AAA').first
+    assert_equal 'AAA', widget.sku
+    assert_equal 0, @store.scan_count
+  end
+
+  def test_find_by_id_for_a_nonexistent_row_never_scans
+    @store.reset_counts!
+    assert_nil IdLookupWidget.find_by(id: @a + 999)
+    assert_equal 0, @store.scan_count
+  end
+end
+
+# Same shape as CountingFkIndexStore (test_orm_has_many_foreign_key_index.rb)
+# plus a counter specifically for id-equality hits, since #lookup_by_index is
+# the one method under test here rather than #lookup_by_index_in.
+class CountingIdIndexStore < Prism::Orm::Store
+  attr_reader :scan_count, :id_index_hits
+
+  def initialize(*args)
+    super
+    @scan_count = 0
+    @id_index_hits = 0
+  end
+
+  def all(table)
+    @scan_count += 1
+    super
+  end
+
+  def lookup_by_index(table, equalities)
+    result = super
+    @id_index_hits += 1 if !result.nil? && equalities.key?('id')
+    result
+  end
+
+  def reset_counts!
+    @scan_count = 0
+    @id_index_hits = 0
+  end
+end