Allow custom ImportResult subclass via :result_class option

Adds a :result_class option to active_admin_import so users can plug in
their own ImportResult subclass to collect adapter-specific data (like
inserted ids) without forcing the gem to take on cross-adapter quirks.

The DSL action block now receives result, options via instance_exec
(matching DEFAULT_RESULT_PROC's signature) so the captured data is
actually reachable from user code. Existing zero-arity blocks are
unaffected since Ruby blocks ignore extra arguments.

A PG-only spec demonstrates the canonical PR #191 use case (capturing
result.ids), which is reliable on PostgreSQL via RETURNING but not on
MySQL/SQLite without extra setup.

Author: Fivell

README.md

@@ -68,6 +68,7 @@ Tool                    | Description
 :timestamps             |bool, tells activerecord-import to not add timestamps (if false) even if record timestamps is disabled in ActiveRecord::Base
 :template               |custom template rendering
 :template_object        |object passing to view
+:result_class           |custom `ImportResult` subclass to collect data from each batch (e.g. inserted ids). Must respond to `add(batch_result, qty)` plus the readers used in flash messages (`failed`, `total`, `imported_qty`, `imported?`, `failed?`, `empty?`, `failed_message`).
 :resource_class         |resource class name
 :resource_label         |resource label value
 :plural_resource_label  |pluralized resource label value (default config.plural_resource_label)
@@ -77,6 +78,37 @@ Tool                    | Description
 
 
 
+#### Custom ImportResult
+
+To collect extra data from each batch (for example the ids of inserted rows so you can enqueue background jobs against them), pass a subclass of `ActiveAdminImport::ImportResult` via `:result_class`:
+
+```ruby
+class ImportResultWithIds < ActiveAdminImport::ImportResult
+  attr_reader :ids
+
+  def initialize
+    super
+    @ids = []
+  end
+
+  def add(batch_result, qty)
+    super
+    @ids.concat(Array(batch_result.ids))
+  end
+end
+
+ActiveAdmin.register Author do
+  active_admin_import result_class: ImportResultWithIds do |result, options|
+    EnqueueAuthorsJob.perform_later(result.ids) if result.imported?
+    instance_exec(result, options, &ActiveAdminImport::DSL::DEFAULT_RESULT_PROC)
+  end
+end
+```
+
+The action block is invoked via `instance_exec` with `result` and `options` as block arguments, so you can either capture them with `do |result, options|` or read them as locals when no arguments are declared.
+
+Note: which batch-result attributes are populated depends on the database adapter and the import options. `activerecord-import` returns ids reliably on PostgreSQL; on MySQL/SQLite the behavior depends on the adapter and options like `on_duplicate_key_update`. Putting the collection logic in your own subclass keeps these adapter quirks in your application code.
+
 #### Wiki
 
 [Check various examples](https://github.com/activeadmin-plugins/active_admin_import/wiki)

lib/active_admin_import/dsl.rb

@@ -104,7 +104,7 @@ def active_admin_import(options = {}, &block)
           result = @importer.import
 
           if block_given?
-            instance_eval(&block)
+            instance_exec result, options, &block
           else
             instance_exec result, options, &DEFAULT_RESULT_PROC
           end

lib/active_admin_import/importer.rb

@@ -18,7 +18,8 @@ class Importer
       :headers_rewrites,
       :batch_size,
       :batch_transaction,
-      :csv_options
+      :csv_options,
+      :result_class
     ].freeze
 
     def initialize(resource, model, options)
@@ -29,7 +30,7 @@ def initialize(resource, model, options)
     end
 
     def import_result
-      @import_result ||= ImportResult.new
+      @import_result ||= (options[:result_class] || ImportResult).new
     end
 
     def file

lib/active_admin_import/options.rb

@@ -16,6 +16,7 @@ module Options
       :ignore,
       :template,
       :template_object,
+      :result_class,
       :resource_class,
       :resource_label,
       :plural_resource_label,

spec/import_spec.rb

@@ -689,4 +689,46 @@ def active_admin_import_context
       end
     end
   end
+
+  # PG-only: activerecord-import populates `result.ids` reliably on PostgreSQL
+  # via RETURNING. On MySQL/SQLite the array is not populated by default, so
+  # the assertion would not be meaningful there. The :result_class option
+  # itself works on every adapter — this spec just demonstrates the canonical
+  # PR #191 use case (collecting inserted ids) on the adapter that supports it.
+  if ENV['DB'] == 'postgres'
+    context 'with custom result_class (PostgreSQL)' do
+      # Subclass that captures inserted ids alongside the standard counters.
+      # Lives in user-land so the gem itself stays free of adapter-specific
+      # quirks around RETURNING. This is the example documented in the README.
+      class ImportResultWithIds < ActiveAdminImport::ImportResult
+        attr_reader :ids
+
+        def initialize
+          super
+          @ids = []
+        end
+
+        def add(batch_result, qty)
+          super
+          @ids.concat(Array(batch_result.ids))
+        end
+      end
+
+      before do
+        add_author_resource(result_class: ImportResultWithIds) do |result, _options|
+          # Expose the captured ids on the flash so the test asserts via the
+          # rendered page rather than closure capture.
+          flash[:notice] = "Imported ids: [#{result.ids.sort.join(',')}]"
+        end
+        visit '/admin/authors/import'
+        upload_file!(:authors)
+      end
+
+      it 'collects the ids of inserted records via the custom subclass' do
+        expect(Author.count).to eq(2)
+        expected = "Imported ids: [#{Author.pluck(:id).sort.join(',')}]"
+        expect(page).to have_content(expected)
+      end
+    end
+  end
 end