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package oracle.toplink.essentials.internal.indirection;

import oracle.toplink.essentials.queryframework.*;
import oracle.toplink.essentials.exceptions.*;
import oracle.toplink.essentials.internal.sessions.AbstractRecord;
import oracle.toplink.essentials.internal.sessions.AbstractSession;

/**
 * QueryBasedValueHolder wraps a database-stored object and
 * implements behavior to access it. The object is read from
 * the database by invoking a user-specified query.
 *
 * @see        ObjectLevelReadQuery
 * @author    Dorin Sandu
 */
public class QueryBasedValueHolder extends DatabaseValueHolder {

    /**
     * Stores the query to be executed.
     */
    protected transient ReadQuery query;

    /**
     * Store the uow identity so that it can be used to determine new transaction logic
     */
    /**
     * Initialize the query-based value holder.
     */
    public QueryBasedValueHolder(ReadQuery query, AbstractRecord row, AbstractSession session) {
        this.row = row;
        this.session = session;

        // Make sure not to put a ClientSession or IsolatedClientSession in
        // the shared cache (indirectly).
        // Skip this if unitOfWork, for we use session.isUnitOfWork() to implement
        // isTransactionalValueholder(), saving us from needing a boolean instance variable.
        // If unitOfWork this safety measure is deferred until merge time with
        // releaseWrappedValuehHolder.
        // Note that if isolated session & query will return itself, which is safe
        // for if isolated it will not go in the shared cache.
        if (!session.isUnitOfWork()) {
            this.session = session.getRootSession(query);
        }
        this.query = query;
    }

    /**
     * Return the query.
     */
    protected ReadQuery getQuery() {
        return query;
    }

    protected Object instantiate() throws DatabaseException {
        return instantiate(getSession());
    }

    /**
     * Instantiate the object by executing the query on the session.
     */
    protected Object instantiate(AbstractSession session) throws DatabaseException {
        if (session == null){
            throw ValidationException.instantiatingValueholderWithNullSession();
        }
        return session.executeQuery(getQuery(), getRow());
    }

    /**
     * Triggers UnitOfWork valueholders directly without triggering the wrapped
     * valueholder (this).
     * 

* When in transaction and/or for pessimistic locking the UnitOfWorkValueHolder * needs to be triggered directly without triggering the wrapped valueholder. * However only the wrapped valueholder knows how to trigger the indirection, * i.e. it may be a batchValueHolder, and it stores all the info like the row * and the query. * Note: This method is not thread-safe. It must be used in a synchronizaed manner */ public Object instantiateForUnitOfWorkValueHolder(UnitOfWorkValueHolder unitOfWorkValueHolder) { return instantiate(unitOfWorkValueHolder.getUnitOfWork()); } /** * Releases a wrapped valueholder privately owned by a particular unit of work. *

* When unit of work clones are built directly from rows no object in the shared * cache points to this valueholder, so it can store the unit of work as its * session. However once that UnitOfWork commits and the valueholder is merged * into the shared cache, the session needs to be reset to the root session, ie. * the server session. */ public void releaseWrappedValueHolder() { AbstractSession session = getSession(); if ((session != null) && session.isUnitOfWork()) { setSession(session.getRootSession(query)); } } /** * Reset all the fields that are not needed after instantiation. */ protected void resetFields() { super.resetFields(); setQuery(null); } /** * Set the query. */ protected void setQuery(ReadQuery theQuery) { query = theQuery; } /** * INTERNAL: * Answers if this valueholder is a pessimistic locking one. Such valueholders * are special in that they can be triggered multiple times by different * UnitsOfWork. Each time a lock query will be issued. Hence even if * instantiated it may have to be instantiated again, and once instantatiated * all fields can not be reset. *

* Since locks will be issued each time this valueholder is triggered, * triggering this directly on the session in auto commit mode will generate * an exception. This only UnitOfWorkValueHolder's wrapping this can trigger * it. * Note: This method is not thread-safe. It must be used in a synchronizaed manner */ public boolean isPessimisticLockingValueHolder() { // Get the easy checks out of the way first. if ((getQuery() == null) || !getQuery().isObjectLevelReadQuery()) { return false; } ObjectLevelReadQuery query = (ObjectLevelReadQuery)getQuery(); // Note even if the reference class is not locked, but the valueholder query // has joined attributes, then this may count as a lock query. // This means it is possible to trigger a valueholder to get an object which // is not to be pess. locked and get an exception for triggering it on the // session outside a transaction. return query.isLockQuery(getSession()); } }





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