isUnique method
Whether columns is covered by a unique index or constraint that holds
for every row.
A filtered index — WHERE IsDeleted = 0 — does not count. It
guarantees uniqueness only among the rows it covers, so treating one as a
unique key would generate a find that can return the wrong row and a
one-to-one relation that can have two children.
Implementation
bool isUnique(List<String> columns) =>
uniqueKeyFor(columns, requireUnconditional: true) != null;