mungedb-aggregate/lib/pipeline/documentSources/CursorDocumentSource.js

var CursorDocumentSource = module.exports = (function(){
	// CONSTRUCTOR
    /**
     * Constructs and returns Documents from the objects produced by a supplied Cursor.
     * An object of this type may only be used by one thread, see SERVER-6123.
	 * 
     * This is usually put at the beginning of a chain of document sources
     * in order to fetch data from the database.
     * 
	 * @class CursorDocumentSource
	 * @namespace munge.pipeline.documentsource
	 * @module munge
	 * @constructor
     * @param	{CursorDocumentSource.CursorWithContext}	cursorWithContext the cursor to use to fetch data
    **/
	var klass = module.exports = CursorDocumentSource = function CursorDocumentSource(cursorWithContext/*, pExpCtx*/){
		base.call(this/*, pExpCtx*/);
		
		this.current = null;
		
//		this.ns = null;
//		/*
//		The bson dependencies must outlive the Cursor wrapped by this
//		source.  Therefore, bson dependencies must appear before pCursor
//		in order cause its destructor to be called *after* pCursor's.
//		*/
//		this.pQuery = null;
//		this.pSort = null;
		
		this._projection = null;
		
		this._cursorWithContext = cursorWithContext;
		if (!this._cursorWithContext || !this._cursorWithContext._cursor){
			throw new Error("CursorDocumentSource requires a valid cursor");
		}
		
	}, base = require('./DocumentSource'), proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});
	
	
	// DEPENDENCIES
	//var Document = require("../Document");
	
	klass.CursorWithContext = (function (){
		/**
         * Holds a Cursor and all associated state required to access the cursor.  An object of this
         * type may only be used by one thread.
        **/
		var klass = function CursorWithContext(){
			this._cursor = null;
		};
		
		return klass;
	})();

	/**
     * Release the Cursor and the read lock it requires, but without changing the other data.
     * Releasing the lock is required for proper concurrency, see SERVER-6123.  This
     * functionality is also used by the explain version of pipeline execution.
	 * 
	 * @method	dispose
	**/
	proto.dispose = function dispose() {
        this._cursorWithContext = null;
	};
		
//	/**
//     * Record the namespace.  Required for explain.
//	 * 
//	 * @method	setNamespace
//	 * @param	{String}	ns	the namespace
//	**/
//	proto.setNamespace = function setNamespace(ns) {}
//
//	/**
//	 * Record the query that was specified for the cursor this wraps, if any.
//	 * This should be captured after any optimizations are applied to
//	 * the pipeline so that it reflects what is really used.
//	 * This gets used for explain output.
//	 * 
//	 * @method	setQuery
//	 * @param	{Object}	pBsonObj	the query to record
//	**/
//	proto.setQuery = function setQuery(pBsonObj) {};
//
//
//	/**
//	 * Record the sort that was specified for the cursor this wraps, if any.
//	 * This should be captured after any optimizations are applied to
//	 * the pipeline so that it reflects what is really used.
//	 * This gets used for explain output.
//	 * 
//	 * @method	setSort
//	 * @param	{Object}	pBsonObj	the query to record
//	**/
//	proto.setSort = function setSort(pBsonObj) {};
	
	/**
	 * setProjection method
	 * 
	 * @method	setProjection
	 * @param	{Object}	projection
	**/
	proto.setProjection = function setProjection(projection) {
		
		if (this._projection){
			throw new Error("projection is already set");
		}
		
		
		//dont think we need this yet
		
//		this._projection = new Projection();
//		this._projection.init(projection);
//		
//        this.cursor().fields = this._projection;

		this._projection = projection;  //just for testing
	};
	
	//----------------virtuals from DocumentSource--------------
    /**
     * Is the source at EOF?
     * 
     * @method	eof
    **/
    proto.eof = function eof() {
        /* if we haven't gotten the first one yet, do so now */
        if (!this.current){
            this.findNext();
        }

        return (this.current === null);
    };
    /**
     * Advance the state of the DocumentSource so that it will return the next Document.  
     * The default implementation returns false, after checking for interrupts. 
     * Derived classes can call the default implementation in their own implementations in order to check for interrupts.
     * 
     * @method	advance
     * @returns	{Boolean}	whether there is another document to fetch, i.e., whether or not getCurrent() will succeed.  This default implementation always returns false.
    **/
    proto.advance = function advance() {
        base.prototype.advance.call(this); // check for interrupts
        
        /* if we haven't gotten the first one yet, do so now */
        if (!this.current){
            this.findNext();
        }

        this.findNext();
        return (this.current !== null);
    };
    /**
     * some implementations do the equivalent of verify(!eof()) so check eof() first
     * 
     * @method	getCurrent
     * @returns	{Document}	the current Document without advancing
    **/
    proto.getCurrent = function getCurrent() {
        /* if we haven't gotten the first one yet, do so now */
        if (!this.current){
            this.findNext();
        }

        return this.current;
    };
    /**
     * Set the underlying source this source should use to get Documents
     * from.
     * It is an error to set the source more than once.  This is to
     * prevent changing sources once the original source has been started;
     * this could break the state maintained by the DocumentSource.
     * This pointer is not reference counted because that has led to
     * some circular references.  As a result, this doesn't keep
     * sources alive, and is only intended to be used temporarily for
     * the lifetime of a Pipeline::run().
     * 
     * @method	setSource
     * @param	{DocumentSource}	pSource	the underlying source to use
    **/
    proto.setSource = function setSource(pTheSource) {
		throw new Error("CursorDocumentSource doesn't take a source");
    };
    /**
     * Create an object that represents the document source.  The object
     * will have a single field whose name is the source's name.  This
     * will be used by the default implementation of addToBsonArray()
     * to add this object to a pipeline being represented in BSON.
     * 
     * @method	sourceToJson
     * @param	{Object} pBuilder	BSONObjBuilder: a blank object builder to write to
     * @param	{Boolean}	explain	create explain output
    **/
    proto.sourceToJson = function sourceToJson(pBuilder, explain) {

        /* this has no analog in the BSON world, so only allow it for explain */
        if (explain)
        {
			//we are not currently supporting explain in munge
        }
    };

	//----------------private--------------
	
	proto.findNext = function findNext(){

        if ( !this._cursorWithContext ) {
            this.current = null;
            return;
        }

        for( ; this.cursor().ok(); this.cursor().advance() ) {

            //yieldSometimes();
//            if ( !this.cursor().ok() ) {
//                // The cursor was exhausted during the yield.
//                break;
//            }

//            if ( !this.cursor().currentMatches() || this.cursor().currentIsDup() )
//                continue;

			
            // grab the matching document
            var documentObj;
//          if (this.canUseCoveredIndex()) { ...  Dont need any of this, I think

			documentObj = this.cursor().current();
			this.current = documentObj;
            this.cursor().advance();
            return;
        }

        // If we got here, there aren't any more documents.
        // The CursorWithContext (and its read lock) must be released, see SERVER-6123.
        this.dispose();
        this.current = null;
	};

	proto.cursor = function cursor(){
        if( this._cursorWithContext && this._cursorWithContext._cursor){
			return this._cursorWithContext._cursor;
        }
        throw new Error("cursor not defined");
	};
	
//	proto.chunkMgr = function chunkMgr(){};

//	proto.canUseCoveredIndex = function canUseCoveredIndex(){};
	
//	proto.yieldSometimes = function yieldSometimes(){};
	
	
	return klass;
})();