Code reviews are an ubiquitous and essential part of the software development process. They also offer a unique, at-scale opportunity for teaching developers in the context of their day-to-day development activities versus something more removed and formal, like a class. Yet there is little research on effective teaching through code reviews: focusing on learning for the author and not just changes to the code. We address this gap through a case study at Google: interviews with 14 developers revealed 12 patterns and 15 anti-patterns in code reviews that impact learning. For instance, explanatory rationale, sample solutions backed by standards, and a constructive tone facilitates learning, whereas harsh comments, excessive shallow critiques, and nonpragmatic reviewing that ignores authors’ constraints hinders learning. We validated our qualitative findings through member checking, interviews with code reviewers, a literature review, and a survey of 324 developers. This comprehensive study provides an empirical evidence of how social dynamics in code reviews impact learning. Based on our findings, we provide practical recommendations on how to frame constructive reviews to create a supportive learning environment.