Add query transformation callback feature

Implementation of query transformation requested in ticket #66:
* Add query_transformation.h with internal types
* Add method session::set_query_transformation<T>() - assigns
user-defined function or functor
* Add session::get_query() - returns string with transformed query.
* Replace use of session::get_query_stream() with get_query()
* Keep session::get_query_stream() to access original query accumulated
in current session statement
* Add test_query_transformation to common tests.
Tested using VS2012 on Windows 8 against DB2, ODBC (PostgreSQL, SQL
Server), MySQL, Oracle, SQLite3.

src/core/query_transformation.h

@@ -0,0 +1,49 @@
+//
+// Copyright (C) 2013 Mateusz Loskot <mateusz@loskot.net>
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef SOCI_QUERY_TRANSFORMATION_H_INCLUDED
+#define SOCI_QUERY_TRANSFORMATION_H_INCLUDED
+
+#include "soci-config.h"
+#include <functional>
+#include <string>
+
+namespace soci
+{
+
+namespace details
+{
+
+// TODO: use std::unary_function<const std::string&, std::string>
+class query_transformation_function : public std::unary_function<std::string, std::string>
+{
+public:
+    virtual result_type operator()(argument_type a) const = 0;
+};
+
+template <typename T>
+class query_transformation : public query_transformation_function
+{
+public:
+    query_transformation(T callback)
+        : callback_(callback)
+    {}
+
+    result_type operator()(argument_type query) const
+    {
+        return callback_(query);
+    }
+
+private:
+    T callback_;
+};
+
+} // namespace details
+
+} // namespace soci
+
+#endif // SOCI_QUERY_TRANSFORMATION_H_INCLUDED

src/core/ref-counted-prepare-info.cpp

@@ -42,5 +42,5 @@ void ref_counted_prepare_info::final_action()
 
 std::string ref_counted_prepare_info::get_query() const
 {
-    return session_.get_query_stream().str();
+    return session_.get_query();
 }

src/core/ref-counted-statement.cpp

@@ -23,7 +23,7 @@ void ref_counted_statement::final_action()
     try
     {
         st_.alloc();
-        st_.prepare(session_.get_query_stream().str(), st_one_time_query);
+        st_.prepare(session_.get_query(), st_one_time_query);
         st_.define_and_bind();
 
         const bool gotData = st_.execute(true);

src/core/session.cpp

@@ -10,6 +10,7 @@
 #include "connection-pool.h"
 #include "soci-backend.h"
 #include "backend-loader.h"
+#include "query_transformation.h"
 
 #ifdef _MSC_VER
 #pragma warning(disable:4355)
@@ -48,15 +49,15 @@ void ensureConnected(session_backend * backEnd)
 } // namespace anonymous
 
 session::session()
-    : once(this), prepare(this), logStream_(NULL),
+    : once(this), prepare(this), query_transformation_(0), logStream_(NULL),
       lastFactory_(NULL), uppercaseColumnNames_(false), backEnd_(NULL),
       isFromPool_(false), pool_(NULL)
 {
 }
 
 session::session(backend_factory const & factory,
     std::string const & connectString)
-    : once(this), prepare(this), logStream_(NULL),
+    : once(this), prepare(this), query_transformation_(0), logStream_(NULL),
       lastFactory_(&factory), lastConnectString_(connectString),
       uppercaseColumnNames_(false),
       isFromPool_(false), pool_(NULL)
@@ -66,7 +67,7 @@ session::session(backend_factory const & factory,
 
 session::session(std::string const & backendName,
     std::string const & connectString)
-    : once(this), prepare(this), logStream_(NULL),
+    : once(this), prepare(this), query_transformation_(0), logStream_(NULL),
       uppercaseColumnNames_(false),
       isFromPool_(false), pool_(NULL)
 {
@@ -79,7 +80,7 @@ session::session(std::string const & backendName,
 }
 
 session::session(std::string const & connectString)
-    : once(this), prepare(this), logStream_(NULL),
+    : once(this), prepare(this), query_transformation_(0), logStream_(NULL),
       uppercaseColumnNames_(false),
       isFromPool_(false), pool_(NULL)
 {
@@ -115,6 +116,7 @@ session::~session()
     }
     else
     {
+        delete query_transformation_;
         delete backEnd_;
     }
 }
@@ -260,6 +262,20 @@ std::ostringstream & session::get_query_stream()
     }
 }
 
+std::string session::get_query() const
+{
+    // preserve logical constness of get_query,
+    // stream used as read-only here, 
+    session* pthis = const_cast<session*>(this);
+
+    // sole place where any user-defined query transformation is applied
+    if (query_transformation_)
+    {
+        return (*query_transformation_)(pthis->get_query_stream().str());
+    }
+    return pthis->get_query_stream().str();
+}
+
 void session::set_log_stream(std::ostream * s)
 {
     if (isFromPool_)

src/core/session.h

@@ -9,6 +9,8 @@
 #define SOCI_SESSION_H_INCLUDED
 
 #include "once-temp-type.h"
+#include "query_transformation.h"
+
 // std
 #include <cstddef>
 #include <ostream>
@@ -62,6 +64,14 @@ class SOCI_DECL session
     details::once_temp_type operator<<(T const & t) { return once << t; }
 
     std::ostringstream & get_query_stream();
+    std::string get_query() const;
+
+    template <typename T>
+    void set_query_transformation(T callback)
+    {
+        delete query_transformation_;
+        query_transformation_= new details::query_transformation<T>(callback);
+    }
 
     // support for basic logging
     void set_log_stream(std::ostream * s);
@@ -107,6 +117,7 @@ class SOCI_DECL session
     session& operator=(session const &);
 
     std::ostringstream query_stream_;
+    details::query_transformation_function* query_transformation_;
 
     std::ostream * logStream_;
     std::string lastQuery_;

src/core/test/common-tests.h

@@ -307,6 +307,7 @@ class common_tests
         test30();
         test31();
         test_get_affected_rows();
+        test_query_transformation();
         test_pull5();
         test_issue67();
     }
@@ -3564,6 +3565,52 @@ void test31()
     std::cout << "test 31 passed\n";
 }
 
+// Issue 66 - test query transformation callback feature
+static std::string lower_than_g(std::string query)
+{
+    return query + " WHERE c < 'g'";
+}
+
+struct greater_than_g_lower_than_j : std::unary_function<std::string, std::string>
+{
+    result_type operator()(argument_type query) const
+    {
+        return query + " WHERE c > 'g' AND c < 'j'";
+    }
+};
+
+void test_query_transformation()
+{
+    session sql(backEndFactory_, connectString_);
+    {
+        // create and populate the test table
+        auto_table_creator tableCreator(tc_.table_creator_1(sql));
+
+        char c;
+        for (c = 'a'; c <= 'z'; ++c)
+        {
+            sql << "insert into soci_test(c) values(\'" << c << "\')";
+        }
+
+        char const* query = "select count(*) from soci_test";
+        {
+            sql.set_query_transformation(lower_than_g);
+            int count;
+            sql << query, into(count);
+
+            assert(count == 'g' - 'a');
+        }
+        {
+            sql.set_query_transformation(greater_than_g_lower_than_j());
+            int count;
+            sql << query, into(count);
+
+            assert(count == 'j' - 'h');
+        }
+    }
+    std::cout << "test test_query_transformation passed" << std::endl;
+}
+
 // Originally, submitted to SQLite3 backend and later moved to common test.
 // Test commit b394d039530f124802d06c3b1a969c3117683152
 // Author: Mika Fischer <mika.fischer@zoopnet.de>