Add a collector to grab the bind values off the AST

Now that the bind values are being stored on the actual AST, we need a
way to pull them off into the array that we were previously maintaining
separately. This requires a full walk of the AST. This is an expensive
operation, so I've also added a visitor for delegating to more than one
visitor in a single pass.

lib/arel/collectors/bind.rb

+# frozen_string_literal: true
+
+module Arel
+  module Collectors
+    class Bind
+      def initialize
+        @binds = []
+      end
+
+      def << str
+        self
+      end
+
+      def add_bind bind
+        @binds << bind
+        self
+      end
+
+      def value
+        @binds
+      end
+    end
+  end
+end

lib/arel/collectors/composite.rb

+# frozen_string_literal: true
+
+module Arel
+  module Collectors
+    class Composite
+      def initialize(left, right)
+        @left = left
+        @right = right
+      end
+
+      def << str
+        left << str
+        right << str
+        self
+      end
+
+      def add_bind bind, &block
+        left.add_bind bind, &block
+        right.add_bind bind, &block
+        self
+      end
+
+      def value
+        [left.value, right.value]
+      end
+
+      protected
+
+      attr_reader :left, :right
+    end
+  end
+end

test/collectors/test_bind.rb

+# frozen_string_literal: true
+require 'helper'
+
+require 'arel/collectors/bind'
+
+module Arel
+  module Collectors
+    class TestBind < Arel::Test
+      def setup
+        @conn = FakeRecord::Base.new
+        @visitor = Visitors::ToSql.new @conn.connection
+        super
+      end
+
+      def collect node
+        @visitor.accept(node, Collectors::Bind.new)
+      end
+
+      def compile node
+        collect(node).value
+      end
+
+      def ast_with_binds bvs
+        table = Table.new(:users)
+        manager = Arel::SelectManager.new table
+        manager.where(table[:age].eq(Nodes::BindParam.new(bvs.shift)))
+        manager.where(table[:name].eq(Nodes::BindParam.new(bvs.shift)))
+        manager.ast
+      end
+
+      def test_compile_gathers_all_bind_params
+        binds = compile(ast_with_binds(["hello", "world"]))
+        assert_equal ["hello", "world"], binds
+
+        binds = compile(ast_with_binds(["hello2", "world3"]))
+        assert_equal ["hello2", "world3"], binds
+      end
+    end
+  end
+end

test/collectors/test_composite.rb

+# frozen_string_literal: true
+require 'helper'
+
+require 'arel/collectors/bind'
+require 'arel/collectors/composite'
+require 'arel/collectors/sql_string'
+
+module Arel
+  module Collectors
+    class TestComposite < Arel::Test
+      def setup
+        @conn = FakeRecord::Base.new
+        @visitor = Visitors::ToSql.new @conn.connection
+        super
+      end
+
+      def collect node
+        sql_collector = Collectors::SQLString.new
+        bind_collector = Collectors::Bind.new
+        collector = Collectors::Composite.new(sql_collector, bind_collector)
+        @visitor.accept(node, collector)
+      end
+
+      def compile node
+        collect(node).value
+      end
+
+      def ast_with_binds bvs
+        table = Table.new(:users)
+        manager = Arel::SelectManager.new table
+        manager.where(table[:age].eq(Nodes::BindParam.new(bvs.shift)))
+        manager.where(table[:name].eq(Nodes::BindParam.new(bvs.shift)))
+        manager.ast
+      end
+
+      def test_composite_collector_performs_multiple_collections_at_once
+        sql, binds = compile(ast_with_binds(["hello", "world"]))
+        assert_equal 'SELECT FROM "users" WHERE "users"."age" = ? AND "users"."name" = ?', sql
+        assert_equal ["hello", "world"], binds
+
+        sql, binds = compile(ast_with_binds(["hello2", "world3"]))
+        assert_equal 'SELECT FROM "users" WHERE "users"."age" = ? AND "users"."name" = ?', sql
+        assert_equal ["hello2", "world3"], binds
+      end
+    end
+  end
+end