Handle when table is already being joined

spec/query_associations_spec.cr

@@ -161,4 +161,19 @@ describe "Query associations" do
 
     result.should eq(line_item_product)
   end
+
+  it "can query with potentially unnecessary joins again" do
+    item = LineItemBox.create
+    product = ProductBox.create
+    line_item_product = LineItemProductBox.create &.line_item_id(item.id).product_id(product.id)
+
+    line_item_query = LineItemQuery.new
+      .id(item.id)
+      .where_line_items_products(LineItemProductQuery.new.id(line_item_product.id))
+    result = ProductQuery.new
+      .where_line_items(line_item_query)
+      .find(product.id)
+
+    result.should eq(product)
+  end
 end

src/avram/query_builder.cr

@@ -274,7 +274,7 @@ class Avram::QueryBuilder
   end
 
   def join(join_clause : Avram::Join::SqlClause)
-    if join_clause.to != table
+    if join_clause.to != table && @joins.none? { |join| join.to == join_clause.to }
       @joins << join_clause
     end
     self