Fix undefined method 'join' for an instance of String on hover (#645)

* create index to reproduce issue

* handle when index[:columns] is Array or String

* users table now has one more index, add that new index to the assertion

* fix lint issues on modified files

* fix another rubocop issue

* sort indexes list when running assertion

Author: cpgo

lib/ruby_lsp/ruby_lsp_rails/hover.rb

@@ -97,7 +97,15 @@ def generate_column_content(name)
           @response_builder.push(
             model[:indexes].map do |index|
               uniqueness = index[:unique] ? " (unique)" : ""
-              "- **#{index[:name]}** (#{index[:columns].join(",")})#{uniqueness}"
+              columns = case index[:columns]
+              when Array
+                index[:columns].join(",")
+              when String
+                index[:columns]
+              else
+                index[:name]
+              end
+              "- **#{index[:name]}** (#{columns})#{uniqueness}"
             end.join("\n"),
             category: :documentation,
           )

test/dummy/db/migrate/20250816140957_add_complex_index_to_users.rb

@@ -0,0 +1,5 @@
+class AddComplexIndexToUsers < ActiveRecord::Migration[8.0]
+  def change
+    execute "CREATE UNIQUE INDEX users_unique_complex ON users (COALESCE(country_id, 0), ltrim(first_name));"
+  end
+end

test/dummy/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2024_10_25_225348) do
+ActiveRecord::Schema[8.0].define(version: 2025_08_16_140957) do
   create_table "composite_primary_keys", primary_key: ["order_id", "product_id"], force: :cascade do |t|
     t.integer "order_id"
     t.integer "product_id"
@@ -55,6 +55,7 @@
     t.datetime "updated_at", null: false
     t.integer "country_id", null: false
     t.boolean "active", default: true, null: false
+    t.index "COALESCE(country_id, 0), ltrim(first_name)", name: "users_unique_complex", unique: true
     t.index ["country_id"], name: "index_users_on_country_id"
   end
 

test/ruby_lsp_rails/hover_test.rb

@@ -321,6 +321,69 @@ class Bar < ApplicationRecord
         CONTENT
       end
 
+      test "handles complex indexes with expressions" do
+        expected_response = {
+          schema_file: "#{dummy_root}/db/schema.rb",
+          columns: [
+            ["id", "integer", nil, false],
+            ["first_name", "string", "", true],
+            ["last_name", "string", nil, true],
+            ["age", "integer", "0", true],
+            ["created_at", "datetime", nil, false],
+            ["updated_at", "datetime", nil, false],
+            ["country_id", "integer", nil, false],
+            ["active", "boolean", "true", false],
+          ],
+          primary_keys: ["id"],
+          foreign_keys: ["country_id"],
+          indexes: [
+            { name: "index_users_on_country_id", columns: ["country_id"], unique: false },
+            { name: "users_unique_complex", columns: "COALESCE(country_id, 0), ltrim(first_name)", unique: true },
+          ],
+        }
+
+        RunnerClient.any_instance.stubs(model: expected_response)
+
+        response = hover_on_source(<<~RUBY, { line: 3, character: 0 })
+          class User < ApplicationRecord
+          end
+
+          User
+        RUBY
+
+        assert_equal(<<~CONTENT.chomp, response.contents.value)
+          ```ruby
+          User
+          ```
+
+          **Definitions**: [fake.rb](file:///fake.rb#L1,1-2,4)
+
+
+          [Schema](#{URI::Generic.from_path(path: dummy_root + "/db/schema.rb")})
+
+          ### Columns
+          - **id**: integer (PK)
+
+          - **first_name**: string - default: ""
+
+          - **last_name**: string
+
+          - **age**: integer - default: 0
+
+          - **created_at**: datetime - not null
+
+          - **updated_at**: datetime - not null
+
+          - **country_id**: integer (FK) - not null
+
+          - **active**: boolean - default: true - not null
+
+          ### Indexes
+          - **index_users_on_country_id** (country_id)
+          - **users_unique_complex** (COALESCE(country_id, 0), ltrim(first_name)) (unique)
+        CONTENT
+      end
+
       private
 
       def hover_on_source(source, position)

test/ruby_lsp_rails/runner_client_test.rb

@@ -58,11 +58,14 @@ class RunnerClientTest < ActiveSupport::TestCase
           ]
         end
         foreign_keys = ["country_id"]
-        indexes = [{ name: "index_users_on_country_id", columns: ["country_id"], unique: false }]
+        indexes = [
+          { name: "users_unique_complex", columns: "COALESCE(country_id, 0), ltrim(first_name)", unique: true },
+          { name: "index_users_on_country_id", columns: ["country_id"], unique: false },
+        ]
         response = @client.model("User") #: as !nil
         assert_equal(columns, response.fetch(:columns))
         assert_equal(foreign_keys, response.fetch(:foreign_keys))
-        assert_equal(indexes, response.fetch(:indexes))
+        assert_equal(indexes.sort_by { |i| i[:name] }, response.fetch(:indexes).sort_by { |i| i[:name] })
         assert_match(%r{db/schema\.rb$}, response.fetch(:schema_file))
       end