We introduce ASketch, the first framework for sketching models in the Alloy language. The Alloy Analyzer is a SAT-based constraint solver that allows users to create valuations for relations with respect to given constraints and bound on the universe of discourse. Alloy users routinely use the valuations to validate their models: enumerate some valuations and inspect them to detect underconstraints or overconstraints. Our key insight is that valid and invalid valuations enable sketching Alloy models where the user writes a partial model with holes and provides some valuations, and the sketching infrastructure completes the model by synthesizing Alloy fragments for the holes. ASketch offers the following extensions to Alloy: (1) it expands the Alloy grammar, allowing users to write holes in an Alloy model; (2) it can parse regular expressions and automatically generate pools of matching fragments to replace the holes; (3) it includes a solver-based technique that encodes the model with holes, the fragments for each hole, and the expected valuations to a meta-model which completes the holes when solved. Experimental results show that ASketch works well for different Alloy models with various number of holes, providing a promising approach to bring the success of traditional program sketching for imperative and functional programs to declarative, relational logic.