Re: Insert policy design question

[Rick Gutleber]

This is from a new file, insertpolicy.h, that is now included in query.h:

class MYSQLPP_EXPORT DefaultInsertPolicy
{
public:
    /// \brief Constructor
    DefaultInsertPolicy();

    /// \brief Destructor
    inline ~DefaultInsertPolicy() {
    }

    /// \brief start a new insert query
    inline void startQuery() {
        sbuffer_.str("");
    }

    /// \brief get the insert query built so far
    inline std::string getQuery() const {
        return sbuffer_.str();
    }

    /// \brief add another object to the insert query
    ///
    /// This method returns true if thre
    template <class SSQLS>
    bool add(const SSQLS & object) {
        if (sbuffer_.str().empty()) {
            sbuffer_ << "INSERT INTO " << object.table() << " ("
<<
                object.field_list() << ") VALUES (";
        } else {
            sbuffer_ << ", (";
        }

        sbuffer_ << object.value_list( ) << ")";
        return true;
    }

    /// \brief can we add another object to the query?
    template <class SSQLS>
    bool canAdd(const SSQLS & object) const {
        return true;
    }

    bool isEmpty() const {
        return sbuffer_.str().empty();
    }

private:
    /// \brief String buffer for storing assembled query
    std::ostringstream sbuffer_;
};


This was added in query.h:

template <class T, class InsertPolicy>
Query& insertfrom(const std::vector<T>& con, InsertPolicy& policy)
{
    reset();

    if (con.begin() == con.end()) {
        return *this;   // empty set!
    }

    policy.startQuery();

    Transaction transaction(*conn_);

    for (typename std::vector<T>::const_iterator it = con.begin();
            it != con.end(); it++) {
        if (policy.canAdd(*it)) {
            policy.add(*it);
        } else {
            if (!exec(policy.getQuery())) {
                transaction.rollback();
                return *this;
            }

            policy.startQuery();

            // if we _still_ can't add, something is seriously wrong
            if (policy.canAdd(*it)) {
                policy.add(*it);
            } else {
                transaction.rollback();
                return *this;
            }
        }
    }

    if ( !policy.isEmpty( ) ) {
        if (!exec(policy.getQuery())) {
            transaction.rollback();
            return *this;
        }
    }

    transaction.commit();
    return *this;
}

Drew M. wrote:
> Yeah send your actual code to the list. :)
>
> On Mon, Oct 27, 2008 at 12:27 PM, Rick Gutleber <rgutleber@stripped 
> <mailto:rgutleber@stripped>> wrote:
>
>     Dear Purveyors and Artisans of MySQL++:
>
>     Recently we discussed the design and implementation of a new
>     feature of the mysqlpp::Query that allows for implementing bulk
>     INSERT statements while providing "policy" objects that help with
>     composing the SQL statements with respect to a maximum SQL query
>     size, maximum number of objects per insert or any other criterion
>     a user could add himself.
>
>     I came up with a prototype that I think is a good design, but when
>     I tried it out I discovered the quoting wasn't right.  It seems I
>     need to use the operator <<( ) method with a stream class (e.g.,
>     mysqlpp::Query) for a SSQLS value_list in order to get quoting for
>     the values that need it.  I had been passing the SSQLS objects to
>     the InsertPolicy object and having it append each value list on to
>     the query, and providing the means to check if a subsequent object
>     can be added in.
>
>     Inside of my DefaultInsertPolicy I am making a VALUES section for
>     each object using the operator <<( ) function on the object's
>     value_list and a std::ostringstream private to the
>     DefaultInsertPolicy object, thinking the quoting would be applied
>     just as it is when you do the same thing with a mysqlpp::Query
>     object (which is done in several places in lib/query.h).
>
>     Can anyone explain what might be the problem?  If more detail is
>     needed I can post the actual code I'm using.
>
>     Rick
>
>
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