add statement cache in connection.

hide methods from api

spec/std/db/dummy_driver.cr

@@ -21,7 +21,7 @@ class DummyDriver < DB::Driver
       @@connections.try &.clear
     end
 
-    def prepare(query)
+    def build_statement(query)
       DummyStatement.new(self, query)
     end
 
@@ -30,6 +30,7 @@ class DummyDriver < DB::Driver
     end
 
     protected def do_close
+      super
     end
   end
 

spec/std/db/statement_spec.cr

@@ -80,4 +80,15 @@ describe DB::Statement do
       stmt.closed?.should be_false
     end
   end
+
+  it "connection should cache statements by query" do
+    with_dummy do |db|
+      rs = db.query "1, ?", 2
+      stmt = rs.statement
+      rs.close
+
+      rs = db.query "1, ?", 4
+      rs.statement.should be(stmt)
+    end
+  end
 end

src/db/connection.cr

@@ -9,17 +9,31 @@ module DB
   #
   # ### Note to implementors
   #
-  # The connection must be initialized in `#initialize` and closed in `#perform_close`.
+  # The connection must be initialized in `#initialize` and closed in `#do_close`.
   #
-  # To allow quering override `#prepare` method in order to return a prepared `Statement`.
-  # Also override `#last_insert_id` to allow safe access to the last inserted id through this connection.
+  # Override `#build_statement` method in order to return a prepared `Statement` to allow querying.
+  # See also `Statement` to define how the statements are executed.
   #
   abstract class Connection
     include Disposable
+    include QueryMethods
+    @statements_cache = {} of String => Statement
 
     # :nodoc:
-    abstract def prepare(query) : Statement
+    def prepare(query) : Statement
+      stmt = @statements_cache.fetch(query, nil)
+      if stmt.is_a?(Nil)
+        stmt = build_statement(query)
+        @statements_cache[query] = stmt
+      end
 
-    include QueryMethods
+      stmt
+    end
+
+    abstract def build_statement(query) : Statement
+
+    protected def do_close
+      @statements_cache.clear
+    end
   end
 end

src/db/result_set.cr

@@ -15,6 +15,7 @@ module DB
   abstract class ResultSet
     include Disposable
 
+    # :nodoc:
     getter statement
 
     def initialize(@statement : Statement)

src/db/statement.cr

@@ -12,6 +12,7 @@ module DB
   abstract class Statement
     include Disposable
 
+    # :nodoc:
     getter connection
 
     def initialize(@connection)