Guard against paged operations usage when using emulated offsets on DB2

The approach used does not work with emulated offsets. At least
the :limit_offset offset_strategy works.

lib/sequel/plugins/paged_operations.rb

@@ -150,6 +150,9 @@ def paged_update(values, opts=OPTS)
         # and return the primary key to operate on as a Sequel::Identifier.
         def _paged_operations_pk(meth)
           raise Error, "cannot use #{meth} if dataset has a limit or offset" if @opts[:limit] || @opts[:offset]
+          if db.database_type == :db2 && db.offset_strategy == :emulate
+            raise Error, "the paged_operations plugin is not supported on DB2 when using emulated offsets, set the :offset_strategy Database option to 'limit_offset' or 'offset_fetch'"
+          end
 
           case pk = model.primary_key
           when Symbol

spec/extensions/paged_operations_spec.rb

@@ -181,6 +181,14 @@
     proc{@ds.limit(1).paged_datasets{}}.must_raise Sequel::Error
   end
 
+  it "should raise error for DB2 with emulated offsets" do
+    @c.dataset = Sequel.mock('db2')[:t]
+    ds = @c.dataset
+    proc{ds.offset(1).paged_delete}.must_raise Sequel::Error
+    proc{ds.offset(1).paged_update(:x=>1)}.must_raise Sequel::Error
+    proc{ds.offset(1).paged_datasets{}}.must_raise Sequel::Error
+  end
+
   it "should raise error for dataset with offset" do
     proc{@ds.offset(1).paged_delete}.must_raise Sequel::Error
     proc{@ds.offset(1).paged_update(:x=>1)}.must_raise Sequel::Error

spec/integration/plugin_test.rb

@@ -3123,4 +3123,4 @@ def set(k, v, ttl) self[k] = v end
     ds.paged_datasets{|ds| counts << ds.count}
     counts.must_equal [49]
   end
-end
+end unless DB.database_type == :db2 && DB.offset_strategy == :emulate