Raise a specific class error instead of string literal (#156)

* Raise a specific class error instead of string literal when the type returned doesn't match the type expected. Allows for drivers to catch the specific error.

* Add ResultSet#next_column_index

* Add shared specs for next_column_index

* Add properties to ColumnTypeMismatchError

* Add shared specs for ColumnTypeMismatchError

* Fix specs

Co-authored-by: Brian J. Cardiff <bcardiff@gmail.com>

spec/custom_drivers_types_spec.cr

@@ -20,6 +20,10 @@ module GenericResultSet
     @index += 1
     @row[@index - 1]
   end
+
+  def next_column_index : Int32
+    @index
+  end
 end
 
 class FooValue
@@ -197,7 +201,7 @@ describe DB do
         FooDriver.fake_row = [1] of FooDriver::Any
         db.query "query" do |rs|
           rs.move_next
-          expect_raises(Exception, "FooResultSet#read returned a Int32. A BarValue was expected.") do
+          expect_raises(DB::ColumnTypeMismatchError, "In FooDriver::FooResultSet#read the column 0 returned a Int32 but a BarValue was expected.") do
             w.check
             rs.read(BarValue)
           end
@@ -210,7 +214,7 @@ describe DB do
         BarDriver.fake_row = [1] of BarDriver::Any
         db.query "query" do |rs|
           rs.move_next
-          expect_raises(Exception, "BarResultSet#read returned a Int32. A FooValue was expected.") do
+          expect_raises(DB::ColumnTypeMismatchError, "In BarDriver::BarResultSet#read the column 0 returned a Int32 but a FooValue was expected.") do
             w.check
             rs.read(FooValue)
           end

spec/dummy_driver.cr

@@ -190,6 +190,10 @@ class DummyDriver < DB::Driver
       return n
     end
 
+    def next_column_index : Int32
+      @column_count - @values.not_nil!.size
+    end
+
     def read(t : String.class)
       read.to_s
     end

src/db/error.cr

@@ -46,4 +46,17 @@ module DB
   # Raised when a scalar query returns no results.
   class NoResultsError < Error
   end
+
+  # Raised when the type returned for the column value
+  # does not match the type expected.
+  class ColumnTypeMismatchError < Error
+    getter column_index : Int32
+    getter column_name : String
+    getter column_type : String
+    getter expected_type : String
+
+    def initialize(*, context : String, @column_index : Int32, @column_name : String, @column_type : String, @expected_type : String)
+      super("In #{context} the column #{column_name} returned a #{column_type} but a #{expected_type} was expected.")
+    end
+  end
 end

src/db/result_set.cr

@@ -69,6 +69,11 @@ module DB
     # Reads the next column value
     abstract def read
 
+    # Returns the column index that corresponds to the next `#read`.
+    #
+    # If the last column of the current row has been read, it must return `#column_count`.
+    abstract def next_column_index : Int32
+
     # Reads the next columns and maps them to a class
     def read(type : DB::Mappable.class)
       type.new(self)
@@ -76,11 +81,18 @@ module DB
 
     # Reads the next column value as a **type**
     def read(type : T.class) : T forall T
+      col_index = next_column_index
       value = read
       if value.is_a?(T)
         value
       else
-        raise "#{self.class}#read returned a #{value.class}. A #{T} was expected."
+        raise DB::ColumnTypeMismatchError.new(
+          context: "#{self.class}#read",
+          column_index: col_index,
+          column_name: column_name(col_index),
+          column_type: value.class.to_s,
+          expected_type: T.to_s
+        )
       end
     end
 

src/spec.cr

@@ -289,6 +289,62 @@ module DB
         ages.should eq([10, 20, 30])
       end
 
+      it "next_column_index" do |db|
+        db.exec sql_create_table_person
+        db.exec sql_insert_person, "foo", 10
+        db.exec sql_insert_person, "bar", 20
+
+        db.query sql_select_person do |rs|
+          rs.move_next
+          rs.next_column_index.should eq(0)
+          rs.read(String)
+          rs.next_column_index.should eq(1)
+          rs.read(Int32)
+          rs.next_column_index.should eq(2)
+
+          rs.move_next
+          rs.next_column_index.should eq(0)
+          rs.read(String)
+          rs.next_column_index.should eq(1)
+          rs.read(Int32)
+          rs.next_column_index.should eq(2)
+        end
+      end
+
+      it "next_column_index when ColumnTypeMismatchError" do |db|
+        db.exec sql_create_table_person
+        db.exec sql_insert_person, "foo", 10
+        db.exec sql_insert_person, "bar", 20
+
+        db.query sql_select_person do |rs|
+          rs.move_next
+          rs.next_column_index.should eq(0)
+          ex = expect_raises(ColumnTypeMismatchError) { rs.read(Int32) }
+          ex.column_index.should eq(0)
+          ex.column_name.should eq("name")
+          # NOTE: sqlite currently returns Int64 due to how Int32 is implemented
+          ex.column_type.should match(/String/)
+          # NOTE: pg currently returns Slice(UInt8) | String due to how String is implemented
+          ex.expected_type.should match(/Int/)
+          rs.next_column_index.should eq(1)
+          ex = expect_raises(ColumnTypeMismatchError) { rs.read(String) }
+          ex.column_index.should eq(1)
+          ex.column_name.should eq("age")
+          # NOTE: sqlite returns Int64
+          ex.column_type.should match(/Int/)
+          # NOTE: pg currently returns Slice(UInt8) | String due to how String is implemented
+          ex.expected_type.should match(/String/)
+          rs.next_column_index.should eq(2)
+
+          rs.move_next
+          rs.next_column_index.should eq(0)
+          expect_raises(ColumnTypeMismatchError) { rs.read(Int32) }
+          rs.next_column_index.should eq(1)
+          expect_raises(ColumnTypeMismatchError) { rs.read(String) }
+          rs.next_column_index.should eq(2)
+        end
+      end
+
       # describe "transactions" do
       it "transactions: can read inside transaction and rollback after" do |db|
         db.exec sql_create_table_person