JsonReader/Record mapping aggregating multiple fields

[Animal]

Jack, on another thread, I've been musing that perhaps the accessor function should just surround the mapping expression with



with (obj) return <mapping expression>;


Then, you'd be able to just use any property names from the row object with no adding of "obj."

How about the following for JsonReader's accessor generator?


/**
* @ignore
*/
getJsonAccessor: function(){
return function(expr) {
var varNameRe = /^\w[\w_]$/;
try {
return(expr.match(varNameRe))
? function(rec){return rec[expr];}
: new Function("rec", "with(rec){return " + expr + "};");
} catch(e){}
return Ext.emptyFn;
};
}(),


So you could just use eg:


mapping: 'firstName + " " + secondName'


To pull data from several different fields of the row object.

[jack.slocum]

Looks promising. Have you tested it for performance? I'd be interested in knowing the results.

[Animal]

Subjectively, paging the grid it seems perhaps faster. Profiling in Firebug can't even catch any CPU time allocated to the function - it say 0ms.

[jack.slocum]

Great! Can I ask what the try/catch is for? It looks like it catching malformed json specs, but wouldn't it make more sense to let those errors go through or throw a nice message?

[Animal]

Yes, it is to catch malformed mappings.

The exception is thrown at the time the accessor function is compiled. That is, on read of the first record (or first record after metadata change)

This works. It shows an explanatory message in each cell of the grid that uses that mapping:



/**
* @ignore
*/
getJsonAccessor: function(){
return function(expr) {
var varNameRe = /^\w[\w_]$/;
try {
return(expr.match(varNameRe))
? function(rec){return rec[expr];}
: new Function("rec", "with(rec){return " + expr + "};");
} catch(e){
return function(){return e.message + ' in mapping "' + expr + '"'};
}
};
}(),

[Animal]

Maybe it should just be even simpler and just use the with() method to always catch these errors:



/**
* @ignore
*/
getJsonAccessor: function(expr) {
try {
return new Function("rec", "with(rec){return " + expr + "};");
} catch(e){
return function(){return e.message + ' in mapping "' + expr + '"'};
}
},

[jack.slocum]

Hmmm... I am liking that last one. Looks nice. One question though, what about a mapping like "col-data". That would bomb right?

[Animal]

Only if either of those properties didn't exist!

If the row object looked like this:



{col: 4, data: 2}


Then it would work fine.

Just paste this into the Firebug console:



var t={col: 4, data: 2};
with(t){col-data};

[jack.slocum]

Yes, I mean the field is called 'col-data'. ;) Like this is valid JSON:

{"foo": "bar", "col-data": 123}

[Animal]

Hmm. Using quotes to get round variable naming rules.

I can't see a justification for doing this, but you could get round it be reintroducing a regexp test to see if



expr.match(/^(['"]).*?\1$/)


And use the simple array notation accessor if it matches.

[Animal]

// private
quotedStr: /^(['"]).*\1$/,

/**
* @ignore
*/
getJsonAccessor: function(expr) {
try {
return(expr.match(this.quotedStr))
? function(rec){return rec[expr];}
: new Function("rec", "with(rec){return " + expr + ";}");
} catch(e){
return function(){return e.message + ' in mapping "' + expr + '"'};
}
},

[manugoel2003]

ok, this thread went right over my head :D .... u ppl are amazing... I feel so obsolete

[zzo]

I need this feature & patched my 2.2 ext-all-debug.js thusly:



quotedStr: /^(['"]).*\1$/,

/**
* @ignore
*/
getJsonAccessor: function(expr) {
var re = /[\[\.]/;
try {
if (expr.match(this.quotedStr)) {
return new Function("rec", "with(rec){return " + expr + ";}");
} else {
if (re.test(expr)) {
return new Function("obj", "return obj." + expr);
} else {
return function(obj){ return obj[expr]; };
}
}
} catch(e){
return function(){return e.message + ' in mapping "' + expr + '"'};
}
},



Seems to work pretty well... :D

[Animal]

Don't patch the main file. This is what Ext.override is for.