diff --git a/test/test_orm_has_many_foreign_key_index.rb b/test/test_orm_has_many_foreign_key_index.rb
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+++ b/test/test_orm_has_many_foreign_key_index.rb
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+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+
+# `has_many`'s generated reader (Associable#has_many, no `through:`/`as:`) builds
+# `Target.where(foreign_key => id)` directly - a single HashNode equality - so it
+# is answerable by Store#lookup_by_index exactly like any other `where(column: value)`
+# once the foreign key column carries an index. Before this index exists, every call
+# to `parent.children` full-scans the child table, even though it is one of the most
+# frequently issued queries in any has_many-shaped app (examples/blog's `post.comments`
+# runs on every single post page load).
+class OrmHasManyForeignKeyIndexTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-fk-index-store')
+    @store = CountingFkIndexStore.new(File.join(@dir, 'test.pstore'))
+    @store.create_table(:parents, %w[id name])
+    @store.create_table(:children, %w[id parent_id label])
+
+    parent_class = Class.new(Prism::Orm::Model) do
+      table_name :parents
+      has_many :children, class_name: 'Child', foreign_key: 'parent_id'
+    end
+    child_class = Class.new(Prism::Orm::Model) { table_name :children }
+    [parent_class, child_class].each { |klass| klass.store = @store }
+    Object.const_set(:FkParent, parent_class)
+    Object.const_set(:Child, child_class)
+
+    @mom = FkParent.create!(name: 'mom')
+    @dad = FkParent.create!(name: 'dad')
+    Child.create!(parent_id: @mom.id, label: 'a')
+    Child.create!(parent_id: @mom.id, label: 'b')
+    Child.create!(parent_id: @dad.id, label: 'c')
+  end
+
+  def teardown
+    FileUtils.remove_entry(@dir)
+    %i[FkParent Child].each { |name| Object.send(:remove_const, name) if Object.const_defined?(name) }
+  end
+
+  def test_has_many_reader_full_scans_without_an_index_on_the_foreign_key
+    @store.reset_counts!
+    labels = @mom.children.to_a.map { |child| child['label'] }.sort
+    assert_equal %w[a b], labels
+    assert_equal 1, @store.scan_count, 'without an index, Target.where(foreign_key => id) must fall back to a scan'
+    assert_equal 0, @store.index_hits
+  end
+
+  def test_has_many_reader_never_scans_once_the_foreign_key_is_indexed
+    @store.add_index(:children, :parent_id)
+
+    @store.reset_counts!
+    labels = @mom.children.to_a.map { |child| child['label'] }.sort
+    assert_equal %w[a b], labels
+    assert_equal 0, @store.scan_count
+    assert_equal 1, @store.index_hits
+
+    @store.reset_counts!
+    dad_labels = @dad.children.to_a.map { |child| child['label'] }
+    assert_equal ['c'], dad_labels
+    assert_equal 0, @store.scan_count
+  end
+
+  def test_indexed_foreign_key_reflects_writes_immediately
+    @store.add_index(:children, :parent_id)
+    Child.create!(parent_id: @mom.id, label: 'd')
+
+    @store.reset_counts!
+    labels = @mom.children.to_a.map { |child| child['label'] }.sort
+    assert_equal %w[a b d], labels
+    assert_equal 0, @store.scan_count
+  end
+end
+
+# `has_many ..., as: :taggable` (Associable#define_has_many_polymorphic) builds a
+# two-column HashNode equality (`likeable_id` + `likeable_type` in examples/blog's
+# `Like`). Store#lookup_by_index only requires an index's own columns to be present
+# in the equalities (Relation#rows always re-applies the full @condition afterward),
+# so a single-column index on just `likeable_id` is already enough to skip the table
+# scan - the leftover `likeable_type` equality is filtered from the (small) candidate
+# set in memory instead of against the whole table. Both shapes are proven below.
+class OrmHasManyPolymorphicForeignKeyIndexTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-fk-poly-index-store')
+    @store = CountingFkIndexStore.new(File.join(@dir, 'test.pstore'))
+    @store.create_table(:posts, %w[id title])
+    @store.create_table(:comments, %w[id body])
+    @store.create_table(:tags, %w[id taggable_id taggable_type label])
+
+    post_class = Class.new(Prism::Orm::Model) do
+      table_name :posts
+      has_many :tags, class_name: 'Tag', as: :taggable
+    end
+    comment_class = Class.new(Prism::Orm::Model) do
+      table_name :comments
+      has_many :tags, class_name: 'Tag', as: :taggable
+    end
+    tag_class = Class.new(Prism::Orm::Model) { table_name :tags }
+    [post_class, comment_class, tag_class].each { |klass| klass.store = @store }
+    Object.const_set(:TaggablePost, post_class)
+    Object.const_set(:TaggableComment, comment_class)
+    Object.const_set(:Tag, tag_class)
+
+    @post = TaggablePost.create!(title: 'hello')
+    @comment = TaggableComment.create!(body: 'hi')
+    Tag.create!(taggable_id: @post.id, taggable_type: 'TaggablePost', label: 'ruby')
+    Tag.create!(taggable_id: @comment.id, taggable_type: 'TaggableComment', label: 'ruby')
+  end
+
+  def teardown
+    FileUtils.remove_entry(@dir)
+    %i[TaggablePost TaggableComment Tag].each { |name| Object.send(:remove_const, name) if Object.const_defined?(name) }
+  end
+
+  def test_without_any_index_the_polymorphic_reader_falls_back_to_a_scan
+    @store.reset_counts!
+    labels = @post.tags.to_a.map { |tag| tag['label'] }
+    assert_equal ['ruby'], labels
+    assert_equal 1, @store.scan_count
+  end
+
+  def test_single_column_index_on_the_id_half_already_avoids_the_scan
+    @store.add_index(:tags, :taggable_id)
+
+    @store.reset_counts!
+    labels = @post.tags.to_a.map { |tag| tag['label'] }
+    assert_equal ['ruby'], labels
+    assert_equal 0, @store.scan_count,
+      'the index narrows candidates by id; the leftover type equality is filtered in memory, not by scanning'
+  end
+
+  def test_composite_index_over_both_columns_avoids_the_scan_and_stays_type_scoped
+    @store.add_index(:tags, %i[taggable_id taggable_type])
+
+    @store.reset_counts!
+    post_labels = @post.tags.to_a.map { |tag| tag['label'] }
+    assert_equal ['ruby'], post_labels
+    assert_equal 0, @store.scan_count
+    assert_equal 1, @store.index_hits
+
+    @store.reset_counts!
+    comment_labels = @comment.tags.to_a.map { |tag| tag['label'] }
+    assert_equal ['ruby'], comment_labels
+    assert_equal 0, @store.scan_count
+  end
+end
+
+# Wraps Store#all/#lookup_by_index so tests can assert on whether a query hit
+# the index or fell back to a full table scan, instead of trusting that it
+# "looks" indexed. Relation#rows only calls #all when indexed_candidates comes
+# back nil, so #all's call count is exactly the full-scan count.
+class CountingFkIndexStore < Prism::Orm::Store
+  attr_reader :scan_count, :index_hits
+
+  def initialize(*args)
+    super
+    @scan_count = 0
+    @index_hits = 0
+  end
+
+  def all(table)
+    @scan_count += 1
+    super
+  end
+
+  def lookup_by_index(table, equalities)
+    result = super
+    @index_hits += 1 unless result.nil?
+    result
+  end
+
+  def reset_counts!
+    @scan_count = 0
+    @index_hits = 0
+  end
+end