Prohibit dropping the last column in a table.

Author: rspeele

src/Rezoom.SQL.Compiler/Error.fs

@@ -143,6 +143,8 @@ let onlyIntPrimaryKeyAutoincrement =
     "SQ060: AUTOINCREMENT can only be specified for an INT or INT64 column"
 let tableAlreadyHasPrimaryKey table =
     sprintf "SQ061: ``%O`` already has ``%O`` as its primary key constraint" table
+let cannotDropLastColumn table columnName =
+    sprintf "SQ062: ``%O`` can't be dropped because it is the last column remaining in table ``%O``" columnName table
 
 
 let tableNameNotSuitableForPG =

src/Rezoom.SQL.Compiler/ModelOps.fs

@@ -303,6 +303,8 @@ let dropColumn tableName (column : Name) =
                             failAt tableName.Source <| Error.columnIsReferencedByConstraints column [refName]
                     | _ -> ()
                 let table = { table with Columns = table.Columns |> Map.remove column }
+                if table.Columns |> Map.isEmpty then
+                    failAt tableName.Source <| Error.cannotDropLastColumn tableName.Value column
                 return! putObject tableName (SchemaTable table)
             else
                 failAt tableName.Source <| Error.columnIsReferencedByConstraints column coveredByConstraints

src/Rezoom.SQL.Test/TestAlterTable.fs

@@ -76,4 +76,11 @@ let ``string can't be autoincrement`` () =
     { defaultTest with
         Migration = "create table Foo(x string(10) primary key autoincrement)"
         Expect = BadMigration <| Error.onlyIntPrimaryKeyAutoincrement
+    } |> assertSimple
+
+[<Test>]
+let ``can't drop last column`` () =
+    { defaultTest with
+        Migration = "create table Foo(x int); alter table Foo drop column x"
+        Expect = BadMigration <| Error.cannotDropLastColumn "main.Foo" "x"
     } |> assertSimple