Support import on datasets with DelayedEvaluation as from

lib/sequel/adapters/shared/postgres.rb

@@ -2430,6 +2430,9 @@ def insert_into_sql(sql)
       # Return the primary key to use for RETURNING in an INSERT statement
       def insert_pk
         (f = opts[:from]) && !f.empty? && (t = f.first)
+
+        t = t.call(self) if t.is_a? Sequel::SQL::DelayedEvaluation
+
         case t
         when Symbol, String, SQL::Identifier, SQL::QualifiedIdentifier
           if pk = db.primary_key(t)

spec/adapters/postgres_spec.rb

@@ -2072,6 +2072,22 @@ def before_create
   end
 end
 
+describe "Postgres::Dataset#import with delayed evaluation as source table" do
+  before do
+    @db = DB
+    @db.create_table!(:test){primary_key :x; Integer :y}
+    @ds = @db[Sequel::SQL::DelayedEvaluation.new(lambda{:test})]
+  end
+  after do
+    @db.drop_table?(:test)
+  end
+
+  it "#import should work correctly when returning primary keys" do
+    @ds.import([:x, :y], [[1, 2], [3, 4]], :return=>:primary_key).must_equal [1, 3]
+    @ds.all.must_equal [{:x=>1, :y=>2}, {:x=>3, :y=>4}]
+  end
+end
+
 describe "Postgres::Dataset#insert" do
   before do
     @db = DB