Add test for findFirst returning null

[atadeuccislideworks]

Hey, this test is actually falling, I tried my hardest to make it work but I don't understand fully the codebase.

    find › findFirst › Should not return item

    expect(received).toEqual(expected) // deep equality

    Expected: null
    Received: {"authorId": 2, "blogId": 2, "createdAt": 2024-01-09T18:59:57.960Z, "id": 2, "imprint": "3e937a1f-cd50-422f-bd0d-624d9ccd441d", "title": "title2"}

      111 |
      112 |       expect(realPost).toEqual(null);
    > 113 |       expect(mockPost).toEqual(null);
          |                        ^
      114 |     });

I think the issue is somewhere near https://github.com/morintd/prismock/blob/3e74dd216317013334b1ca799c504f9bca7ca636/src/lib/operations/find/match.ts#L77

Changing the output of getFieldRelationshipWhere to:

export const getFieldRelationshipWhere = (
  item: Item,
  field: DMMF.Field,
  delegates: Delegates,
): Record<string, GroupByFieldArg> => {
  if (field.relationToFields?.length === 0) {
    field = getJoinField(field, delegates)!;
    return {
      [field.relationFromFields![0]]: item[field.relationToFields![0]] as GroupByFieldArg,
    };
  }
  return {
    [field.relationFromFields![0]]: item[field.relationToFields![0]] as GroupByFieldArg,
  };
};

made my new test pass but broke find-nested.test.ts and some other tests, with this change https://github.com/morintd/prismock/blob/3e74dd216317013334b1ca799c504f9bca7ca636/src/lib/operations/find/match.ts#L79

returns { id: 1, authorId: 2 }, without the change it returns only { id : 2 } because getFieldRelationshipWhere overrides the Object.assign({}, childWhere) since it's the same key id.

@morintd if you have some idea on how to fix the nested find now, I would love to submit it into this PR

[morintd]

Hello @atadeuccislideworks , thanks for submission, I'll have a look in the next few days! 🙏