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CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
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Building a Query
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We re going to define the find method, which builds a query and sends it to the database.
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def find[FT <: Product with FieldProduct[MyType]] (cols: FT, query: QueryParam[MyType]*): List[FT#ReturnType] = { query: QueryParam[MyType]* is the definition of a variable argument list. So, this method can take zero or more QueryParams. We return a List[FT#ReturnType] with a List of the ReturnType dependent type of FT. This is some fairly gnarly stuff. The find method takes FT as a type parameter. FT is a subclass of FieldProduct. FieldProduct has a dependent type called ReturnType, but that type is not stable; it is variable depending on the FT type parameter. Thus, the # rather than the . says, Here s a type that s calculated based on the type
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parameter rather than a type that is stable.
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val select = "SELECT "+cols.fields.map(f => table+"."+f.name).mkString(", ")
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Put a List of the By instance into the by variable.
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val by = query.flatMap{ case b @ By(_, _) => Some(b)
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The case b @ By(_, _) code will match any instance of By and assign it to the b variable.
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case _ => None }.toList val where = by match { case Nil => "" case xs => " WHERE "+xs.map(f => table+"."+ f.column.name+" = ").mkString(" AND ") } val orderBy = query.flatMap{ case b @ OrderBy(_, _) => Some(b) case _ => None }.toList match { case Nil => "" case xs => " ORDER BY "+xs.map(f => table+"."+ f.column.name+ f.order.sql).mkString(", ") }
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With a JDBC connection, we prepare a PreparedStatement and bind the query parameters to the in the WHERE clause. We execute the PreparedStatement and for each row in the ResultSet build a Tuple to return.
CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
using(getJDBCConnection) { conn => prepareStatement(conn, select + where + orderBy) { st => by.zipWithIndex.foreach{case (b, idx) => b.bind(st, idx + 1)} executeQuery(st) { cols.buildResult _ } } } }
We get a JDBC connection:
protected def getJDBCConnection: Connection
We define a series of control structures for running queries. We saw control structures like these in 4.
def using[T <: {def close(): Unit}, R](t: T)(f: T => R): R = try {f(t)} finally {t.close()} protected def prepareStatement[T](conn: Connection, sql: String) (f: PreparedStatement => T): T = using(conn.prepareStatement(sql))(f) protected def executeQuery[T](st: PreparedStatement) (f: ResultSet => T): List[T] = using(st.executeQuery){ rs => val ret = new ListBuffer[T] while (rs.next) ret += f(rs) ret.toList } }
Making Queries Type-Safe
We ve defined our table class. Let s define some QueryParameters. QueryParams are type-safe in that they can only contain fields defined in the Table that we re passing them to. We guarantee this by defining the TableType parameter. In the Table.find method, we only accept QueryParam[MyType], so the compiler will enforce this requirement.
trait QueryParam[TableType <: Table[TableType]]
CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
Next, we define the OrderBy case class. It takes column and direction parameters. The column s TableType equals the OrderBy s TableType, thus we can only use columns from the table that we re running the query on. SortOrder is a sealed trait. Marking a trait or class sealed means that all classes that implement that trait must be defined in this file, ensuring that subclasses don t sneak into our code. We define two case objects, Ascending and Descending, which implement the trait.
case class OrderBy[TableType <: Table[TableType]] (column: BasicColumn[TableType, _], order: SortOrder) extends QueryParam[TableType] sealed trait SortOrder {def sql: String} case object Ascending extends SortOrder {def sql: String = " "} case object Descending extends SortOrder {def sql = " DESC "}
Next, we define the By clause of our query. By takes three type parameters: TableType, T, PT. T is the type of the column. PT is the type of the parameter to the query. column is a BasicColumn[TableType, T]. param is a PT. There s also an implicit parameter, f, which converts an instance of PT into an instance of T. Marking the parameter implicit has two effects. First, when we construct an instance of By, the compiler will look for an implicit conversion in the current scope that turns a PT into a T. If one cannot be found, the compiler will flag an error. Second, within the scope of the By, any time you have a PT and you need a T, the compiler will apply the function. The net result of this is that you can pass an Int as a parameter to a query where the column is a Long, and the compiler won t complain.
case class By[TableType <: Table[TableType], T, PT] (column: BasicColumn[TableType, T], param: PT)(implicit f: PT => T) extends QueryParam[TableType] { def bind(st: PreparedStatement, offset: Int): Unit = { column.set(st, offset, param) } }
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