julia> function not_empty(field::Symbol, m::T)::ValidationResult where {T<:AbstractModel}
         isempty(getfield(m, field)) && return ValidationResult(invalid, :not_empty, "should not be empty")

         ValidationResult(valid)
       end

julia> function is_int(field::Symbol, m::T)::ValidationResult where {T<:AbstractModel}
         isa(getfield(m, field), Int) || return ValidationResult(invalid, :is_int, "should be an int")

         ValidationResult(valid)
       end

julia> function is_unique(field::Symbol, m::T)::ValidationResult where {T<:AbstractModel}
         obj = findone(typeof(m); NamedTuple(field => getfield(m, field))... )
         if ( obj !== nothing && ! ispersisted(m) )
           return ValidationResult(invalid, :is_unique, "already exists")
         end

         ValidationResult(valid)
       end

julia> ValidationRule(:username, not_empty)
julia> ValidationRule(:username, is_unique)
julia> ValidationRule(:age, is_int)
julia> ValidationRule(:email, not_empty)