EntityFunction for Relational Database error

Question

I am a student and I am new of Mathematica. I am working with relational databases and I am following the guide "Relational Database quick start" find here: https://reference.wolfram.com/language/tutorial/RelationalDatabasesQuickStart.html

I have successfully managed to connect to my database. Now I am using the Entity Framework to query my database.

The problem is that I want to pass a global value inside the EntityFunction and not define the column name a priority. I would like to use the code inside a function and pass dynamic parameters to this function.

So this first example works.

EntityValue[
  FilteredEntityClass[
   "Customer",
   EntityFunction[e, e["City"] != "Vancuver"]
   ],
  {"FirstName"},
  "PropertyAssociation"
  ] // Dataset

But I want to do something like this

function[var_,value_] := (
 entity = EntityValue[
  FilteredEntityClass[
   "Customer",
   EntityFunction[e, e[var] != value]
   ],
  {"FirstName"},
  "PropertyAssociation"
  ] // Dataset;
 Return[entity];
);

res = function["City", "Vancouver"];

When I try this second solution I get this error

EntityFunction::invexpr: The expression e[var] cannot be compiled to SQL.

Is it not possible to do something like this?

Since "Vancuver" is typed by "City" in this relational database there is simply no need for this. — Steffen Jaeschke, May 28 '20 at 20:03
I modified the code inside the question because perhaps I had not explained myself well. I would like to pass parameters in EntityFunction. In the code shown if I use only "value" as a parameter of the function everything goes correctly, but if I change it and I also have the parameter "var" I get the error shown. It seem that e[var] is not accepted by Mathematica. — Daniele F, May 28 '20 at 20:49
Your updated example works for me. You might have some left-over earlier definitions of function that are interfering with its operation. Try re-evaluating its definition after evaluating ClearAll[function]. — WReach, May 28 '20 at 23:52

score 2 · Answer 1 · answered May 28 '20 at 20:49

2

Since EntityFunction, just like Function, holds its variable and body, you will have to inject the value into it. This is not specific to EntityFunction, and is a common problem of injecting values into held expressions. Just as in other such cases, you can use e.g. With to do that:

With[{var = "City"},
  EntityValue[
    FilteredEntityClass[
      "Customer",
      EntityFunction[e, e[var] != "Vancuver"]
    ],
    {"FirstName"},
    "PropertyAssociation"
  ] // Dataset
]

answered May 28 '20 at 20:49

Leonid Shifrin

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FYI, the OP has rewritten the question such that your response no longer applies. – WReach May 28 '20 at 23:55
@WReach Thanks for letting me know. I hate when this happens, feel like a fool :). Anyway, I will keep it around for the time being. – Leonid Shifrin May 29 '20 at 15:02

score 0 · Answer 2 · answered May 29 '20 at 08:43

Most probable this

Dataset vw. Association

sheets sufficient help for the intend of the question. Since the answer there goes deeper than this question it hard to name this question a duplicate. It is too under question whether the shown competence on relational data handling from the questioner is sufficient to understand the answer suggested.

Aside from typing scoping is for sure a possible resolution, change of perspective to this question.

EntityFunction for Relational Database error

2 Answers2