arel: add

.projections.json

@@ -0,0 +1,10 @@
+{
+  "*.md": {
+    "type": "cheat",
+    "template": [
+      "---",
+      "title: {basename|capitalize}",
+      "---"
+    ]
+  }
+}

arel.md

@@ -0,0 +1,52 @@
+---
+title: Arel
+---
+
+### Tables
+
+```rb
+users = Arel::Table.new(:users)
+```
+
+### Fields
+
+```rb
+users[:name]
+users[:id]
+```
+
+### `where` (restriction)
+
+```rb
+users.where(users[:name].eq('amy'))
+# SELECT * FROM users WHERE users.name = 'amy'
+```
+
+### `select` (projection)
+
+```rb
+users.project(users[:id])
+# SELECT users.id FROM users
+```
+
+### `join`
+
+```rb
+users.join(photos)
+users.join(photos, Arel::Nodes::OuterJoin).on(users[:id].eq(photos[:user_id]))
+```
+
+### `limit` / `offset`
+
+```rb
+users.take(5) # => SELECT * FROM users LIMIT 5
+users.skip(4) # => SELECT * FROM users OFFSET 4
+```
+
+### Aggregates
+
+```rb
+users.project(users[:age].sum) # .average .minimum .maximum
+users.project(users[:id].count)
+users.project(users[:id].count.as('user_count'))
+```