Add test cases about MySQL ORDER BY FIELD()

[chezou]

These assertions examine the sanitization of ORDER BY FIELD() with empty data on MySQL appropriately.

  Tag.order(['field(id, ?)', []]).to_sql
  # => SELECT "tags".* FROM "tags" ORDER BY field(id, NULL)

  Tag.order(['field(id, ?)', nil]).to_sql
  # => SELECT "tags".* FROM "tags" ORDER BY field(id, NULL)

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[senny]

@chezou what was the reason that prompted you to write this test-case?

[chezou]

These are special cases of ORDER BY FIELD() which return records same as without ORDER BY clause in current implementation of MySQL. In order to clarify what AR returns, I added the tests.

[senny]

@chezou does AR do anything else than returning the records exactly as they are returned from MySQL? I don't understand what behavior of AR we are testing here. If it's something thats dependent on the MySQL implementation then we just pass on whatever we receive.

[pixeltrix]

@senny I think it's more testing the sanitisation of order clauses - I guess a couple of extra tests for empty and nil values wouldn't be harmful, though I hope we cover sanitisation of [] and nil elsewhere as well.

[chezou]

Thank you for merge it. The second and the subsequent arguments are important for FIELDS() function. So this spec makes clear that AR sanitize [] and nil properly.