Q: How to override property lookup logic (support fields with dots)

[evil-shrike]

Hi! I wonder is it possible to override the logic of fields navigation. For example, if I want instead of normal navigation of nested object also support fields with dots in their names. Here's an example, two objects, obj1 - with a nested value, obj2 - with a dot in a field's name. I'd expect the expression "main.temp" works in both cases.

let obj1 = { main: { temp: 280  } }; 
let obj2 = { "main.temp": 280  }; 
let expr = 'main.temp > 273';

Works:

console.log(math.evaluate(expr, obj1));

Doesn't (throws 'Undefined symbol main'):

console.log(math.evaluate(expr, obj2));

[josdejong]

two nested objects is really different from a key with a dot in the name, and it can lead to ambiguous cases when you want to resolve nested objects. Like here:

let obj3 = { 
  main: { 
    temp: 280  
  }, 
  'main.temp': 260 
};

So what would you expect to get when evaluating let expr = 'main.temp > 273'; in this case?

What you maybe could do is pre-process your scope: replace objects with a dot in the name with nested objects. I.e. replace obj2 with obj in your example.

[evil-shrike]

Well, yeah, agree it's not very straightforward, that's because I'm asking about customization and don't expect the lib supports this case by default.

In my case I have several data feeds (arrays of object, possibly nested), that I want to join. For this, I flatten them into array-like maps, like this:

// input

feed1: [
  { city: {name: "city1", id: 1}, main: { temp: 280 }},
  { city: {name: "city2" id: 2}, main: { temp: 281 }}
]

// output

feed1: [
{
 "city.name": "city1",
 "city.id":  1,
  "main.temp": 280
},
{
 "city.name": "city2",
 "city.id":  2,
  "main.temp": 281
}

I'm doing this as I have another feed with a foreign key:

feed2: [
  { "city_id": 1, "some_column": "some_value1" },
  { "city_id": 2, "some_column": "some_value2" }
]

and a result feed will be:

result: [
  {
    "city.name": "city1",
    "city.id": 1,
    "main.temp": 280,
    "some_column": "some_value1"
  }, // etc
]

so I don't have any ambiguity, the expr 'main.temp > 273' will address a "column" (technically a field).

I understand that there're workarounds, e.g. I can possible unwind it back to nested objects:

[
 {
    feed1: { city: { name: "city", id: 1}, main: {temp: 280}, feed2: { some_column: "some_value1"} }, //etc
   // or
    { city: { name: "city", id: 1}, main: {temp: 280}, some_column: "some_value1"}, //etc
  }
]

or try to join them without flattening. But there could be other reason for flattening so I wanted to investigate all my options.

[josdejong]

I understand your use case. I agree that this is a quite specific use case so it makes sense to keep this as a solution (could be a plugin or simply a snippet) separate from mathjs.

I think the most straightforward solution is what you're proposing: do some flattening and merging beforehand yourself to mold the data in the structure that you want to have to expose it inside the expression editor.

[chanand]

The solution I found for a similar problem was to wrap the object like this:

math.evaluate(`data['main.temp'] > 273`, { data })