Is #Mg(NO_3)_2# soluble?
Well, it is soluble in water...............
Standard solubility rules suggest that all nitrates are soluble. Why? Because the nitrate ion is not very charge dense, and the activity of the anion is diminished with respect to other anions. You will have to look up the actual values.
This salt would also have some solubility in alcohols due to the diminished activity of the nitrate. Note that the nitrate anion is a potent oxidant, however, this potential is often masked in aqueous solution.