Foreign Key Validation Does Not Work As Expected

[baalmael]

ForeignKey.validate(Table table) implements something like a foreign key check for self-referencing FK but it does not work correctly.

Let me explain:
In the tests there is this schema (population_schema_for_fk_check.json):

{
  "fields": [
...
  ],
    "foreignKeys": [
    {
      "fields": ["check_city", "check_year"],
      "reference": {
        "resource": "",
        "fields": ["city", "year"]
      }
    }
  ]
}

When we run the test with the following Data the test show a false negative:

False Negative Data:

city	year	population	check_city	check_year
london	2017	8 780 000	paris	2017
paris	2017	2 240 000	paris	2017

Result: io.frictionlessdata.tableschema.exception.ForeignKeyException: Foreign key [check_city-> city] violation : expected: paris found: london

The implementation checks if the value of the foreign key is the same as the reference in the same row which is not how foreign keys work.