Add social graph recipe.

recipes/4_higher_level_data_structures/implement_a_social_graph/meta.yml

@@ -0,0 +1,7 @@
+---
+title: Implement a Simple Social Graph
+credit:
+  - Michael Bleigh
+tags: 
+  - sets
+  - abstractions

recipes/4_higher_level_data_structures/implement_a_social_graph/recipe.md

@@ -0,0 +1,30 @@
+### Problem
+
+You want to use Redis to implement a social graph for users in some kind of application, with one and two directional relationships available (following and friendship).
+
+### Solution
+
+Use the built-in set functionality of Redis to construct `follow`, `follower`, and `blocked` lists keyed to each user's unique ID. In raw redis it looks something like this:
+
+    redis> SADD user:1:follows 2
+    (integer) 1
+    redis> SADD user:2:followers 1
+    (integer) 1
+    redis> SADD user:3:follows 1
+    (integer) 1
+    redis> SADD user:1:followers 3
+    (integer) 1
+    redis> SADD user:1:follows 3
+    (integer) 1
+    redis> SADD user:3:followers 1
+    (integer) 1
+    redis> SINTER user:1:follows user:1:followers
+    1. 3
+
+### Discussion
+
+Redis comes with the ability to construct "sets", which are collections of unique values assigned to a key. By creating both a "follows" and "followers" list for a given user, we are able to quickly and easily pull that information as well as calculate their "friendships" using a simple set intersection.
+
+Implementing such a system in Ruby looks something like this:
+
+{% code_snippet social_graph.rb %}

recipes/4_higher_level_data_structures/implement_a_social_graph/social_graph.rb

@@ -0,0 +1,24 @@
+REDIS = Redis.new
+
+class User
+  def self.find(*ids)
+    # gets a set of User objects given IDs
+  end
+
+  def follow(other_user)
+    REDIS.sadd("user:#{self.id}:follows", other_user.id)
+    REDIS.sadd("user:#{other_user.id}:followers", self.id)
+  end
+
+  def followers
+    User.find(*REDIS.smembers("user:#{self.id}:followers"))
+  end
+
+  def follows
+    User.find(*REDIS.smembers("user:#{self.id}:follows"))
+  end
+
+  def friends
+    User.find(*REDIS.sinter("user:#{self.id}:followers", "user:#{self.id}:follows"))
+  end
+end