Raise error if provided file path cannot be found

This will help developers debug issues related to an incorrect file path as early as possible.

lib/data_migrate.rb

@@ -20,6 +20,6 @@
 
 module DataMigrate
   def self.root
-    File.dirname(__dir__)
+    File.dirname(__FILE__)
   end
 end

lib/data_migrate/config.rb

@@ -22,5 +22,11 @@ def initialize
       @db_configuration = nil
       @spec_name = nil
     end
+
+    def data_template_path=(value)
+      @data_template_path = value.tap do |path|
+        raise ArgumentError, "File not found: '#{path}'" unless path == DEFAULT_DATA_TEMPLATE_PATH || File.exists?(path)
+      end
+    end
   end
 end

spec/data_migrate/config_spec.rb

@@ -39,7 +39,9 @@
       end
     end
 
-    let(:data_template_path) { "lib/awesome/templates/data_migration.rb" }
+    let(:data_template_path) do
+      File.join(DataMigrate.root, "generators", "data_migration", "templates", "data_migration.rb")
+    end
 
     after do
       DataMigrate.configure do |config|
@@ -50,5 +52,15 @@
     it "equals the custom data template path" do
       is_expected.to eq data_template_path
     end
+
+    context "when path does not exist" do
+      subject { DataMigrate.config.data_template_path = invalid_path }
+
+      let(:invalid_path) { "lib/awesome/templates/data_migration.rb" }
+
+      it "checks that file exists on setting config var" do
+        expect { subject }.to raise_error { ArgumentError.new("File not found: '#{data_template_path}'") }
+      end
+    end
   end
 end

spec/generators/data_migration/data_migration_generator_spec.rb

@@ -68,7 +68,7 @@
 
     let(:default_source_root) do
       File.expand_path(
-        File.dirname(File.join(DataMigrate.root, "lib", "generators", "data_migration", "templates", "data_migration.rb"))
+        File.dirname(File.join(DataMigrate.root, "generators", "data_migration", "templates", "data_migration.rb"))
       )
     end
 
@@ -82,8 +82,10 @@
         end
       end
 
-      let(:data_template_path) { "lib/awesome/templates/data_migration.rb" }
-      let(:expected_source_root) { File.expand_path(File.dirname(File.join(DataMigrate.root, data_template_path))) }
+      let(:data_template_path) do
+        File.join(DataMigrate.root, "generators", "data_migration", "templates", "data_migration.rb")
+      end
+      let(:expected_source_root) { File.dirname(data_template_path) }
 
       after do
         DataMigrate.configure do |config|