orm: improve the README and the tests. Add an error message for foreign key tags (#15670)

vlib/v/gen/c/sql.v

@@ -157,8 +157,12 @@ fn (mut g Gen) sql_insert(node ast.SqlStmtLine, expr string, table_name string,
 		} else if sym.kind == .array {
 			mut f_key := ''
 			for attr in f.attrs {
-				if attr.name == 'fkey' && attr.has_arg && attr.kind == .string {
-					f_key = attr.arg
+				if attr.name == 'fkey' && attr.has_arg {
+					if attr.kind == .string {
+						f_key = attr.arg
+					} else {
+						verror("fkey attribute need be string. Try [fkey: '$attr.arg'] instead of [fkey: $attr.arg]")
+					}
 				}
 			}
 			if f_key == '' {
@@ -714,8 +718,12 @@ fn (mut g Gen) sql_select(node ast.SqlExpr, expr string, left string, or_expr as
 			} else if sym.kind == .array {
 				mut fkey := ''
 				for attr in field.attrs {
-					if attr.name == 'fkey' && attr.has_arg && attr.kind == .string {
-						fkey = attr.arg
+					if attr.name == 'fkey' && attr.has_arg {
+						if attr.kind == .string {
+							fkey = attr.arg
+						} else {
+							verror("fkey attribute need be string. Try [fkey: '$attr.arg'] instead of [fkey: $attr.arg]")
+						}
 					}
 				}
 				if fkey == '' {

vlib/v/tests/orm_sub_array_struct_test.v

@@ -1,12 +1,12 @@
 import sqlite
 
 struct Parent {
-	id      int      [primary; sql: serial]
-	name    string
-	chields []Chield [fkey: 'parent_id']
+	id       int     [primary; sql: serial]
+	name     string
+	children []Child [fkey: 'parent_id']
 }
 
-struct Chield {
+struct Child {
 	id        int    [primary; sql: serial]
 	parent_id int
 	name      string
@@ -20,11 +20,11 @@ fn test_orm_array() {
 
 	par := Parent{
 		name: 'test'
-		chields: [
-			Chield{
+		children: [
+			Child{
 				name: 'abc'
 			},
-			Chield{
+			Child{
 				name: 'def'
 			},
 		]
@@ -43,7 +43,7 @@ fn test_orm_array() {
 	}
 
 	assert parent.name == par.name
-	assert parent.chields.len == par.chields.len
-	assert parent.chields[0].name == 'abc'
-	assert parent.chields[1].name == 'def'
+	assert parent.children.len == par.children.len
+	assert parent.children[0].name == 'abc'
+	assert parent.children[1].name == 'def'
 }