Remove `klass` and `arel_table` as a dependency of `PredicateBuilder`

This class cares far too much about the internals of other parts of
Active Record. This is an attempt to break out a meaningful object which
represents the needs of the predicate builder. I'm not fully satisfied
with the name, but the general concept is an object which represents a
table, the associations to/from that table, and the types associated
with it. Many of these exist at the `ActiveRecord::Base` class level,
not as properties of the table itself, hence the need for another
object. Currently it provides these by holding a reference to the class,
but that will likely change in the future. This allows the predicate
builder to remain wholy concerned with building predicates.

/cc @mrgilman

1 files changed, 46 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/table_metadata.rb b/activerecord/lib/active_record/table_metadata.rb
new file mode 100644
index 0000000000..bf705d3565
--- /dev/null
+++ b/activerecord/lib/active_record/table_metadata.rb
@@ -0,0 +1,46 @@
+module ActiveRecord
+  class TableMetadata # :nodoc:
+    delegate :foreign_type, :foreign_key, to: :association, prefix: true
+
+    def initialize(klass, arel_table, association = nil)
+      @klass = klass
+      @arel_table = arel_table
+      @association = association
+    end
+
+    def resolve_column_aliases(hash)
+      hash = hash.dup
+      hash.keys.grep(Symbol) do |key|
+        if klass.attribute_alias? key
+          hash[klass.attribute_alias(key)] = hash.delete key
+        end
+      end
+      hash
+    end
+
+    def arel_attribute(column_name)
+      arel_table[column_name]
+    end
+
+    def associated_with?(association_name)
+      klass && klass._reflect_on_association(association_name)
+    end
+
+    def associated_table(table_name)
+      arel_table = Arel::Table.new(table_name)
+      association = klass._reflect_on_association(table_name)
+      if association && !association.polymorphic?
+        klass = association.klass
+      end
+      TableMetadata.new(klass, arel_table, association)
+    end
+
+    def polymorphic_association?
+      association && association.polymorphic?
+    end
+
+    protected
+
+    attr_reader :klass, :arel_table, :association
+  end
+end