diff --git a/postgres/assignments.sql.go b/postgres/assignments.sql.go
index 8602e7a..52bd45d 100644
--- a/postgres/assignments.sql.go
+++ b/postgres/assignments.sql.go
@@ -17,7 +17,7 @@ INSERT INTO assignments (
 ) VALUES (
         $1, $2, $3
 )
-RETURNING id, category_id, date, memo, amount
+RETURNING category_id, date, memo, amount
 `
 
 type CreateAssignmentParams struct {
@@ -30,7 +30,6 @@ func (q *Queries) CreateAssignment(ctx context.Context, arg CreateAssignmentPara
 	row := q.db.QueryRowContext(ctx, createAssignment, arg.Date, arg.Amount, arg.CategoryID)
 	var i Assignment
 	err := row.Scan(
-		&i.ID,
 		&i.CategoryID,
 		&i.Date,
 		&i.Memo,
@@ -130,3 +129,22 @@ func (q *Queries) GetAssignmentsByMonthAndCategory(ctx context.Context, budgetID
 	}
 	return items, nil
 }
+
+const updateAssignment = `-- name: UpdateAssignment :exec
+INSERT INTO assignments (category_id, date, amount)
+VALUES($1, $2, $3) 
+ON CONFLICT (category_id, date) 
+DO 
+   UPDATE SET amount = $3
+`
+
+type UpdateAssignmentParams struct {
+	CategoryID uuid.UUID
+	Date       time.Time
+	Amount     numeric.Numeric
+}
+
+func (q *Queries) UpdateAssignment(ctx context.Context, arg UpdateAssignmentParams) error {
+	_, err := q.db.ExecContext(ctx, updateAssignment, arg.CategoryID, arg.Date, arg.Amount)
+	return err
+}
diff --git a/postgres/models.go b/postgres/models.go
index 4e33e88..7ff33e8 100644
--- a/postgres/models.go
+++ b/postgres/models.go
@@ -40,7 +40,6 @@ type Account struct {
 }
 
 type Assignment struct {
-	ID         uuid.UUID
 	CategoryID uuid.UUID
 	Date       time.Time
 	Memo       sql.NullString
diff --git a/postgres/queries/assignments.sql b/postgres/queries/assignments.sql
index 56975ef..cb0687d 100644
--- a/postgres/queries/assignments.sql
+++ b/postgres/queries/assignments.sql
@@ -22,4 +22,11 @@ SELECT assignments.date, categories.name as category, category_groups.name as gr
 FROM assignments
 INNER JOIN categories ON categories.id = assignments.category_id
 INNER JOIN category_groups ON categories.category_group_id = category_groups.id
-WHERE category_groups.budget_id = @budget_id;
\ No newline at end of file
+WHERE category_groups.budget_id = @budget_id;
+
+-- name: UpdateAssignment :exec
+INSERT INTO assignments (category_id, date, amount)
+VALUES($1, $2, $3) 
+ON CONFLICT (category_id, date) 
+DO 
+   UPDATE SET amount = $3;
\ No newline at end of file
diff --git a/postgres/schema/0017_natural-key-assignments.sql b/postgres/schema/0017_natural-key-assignments.sql
new file mode 100644
index 0000000..27c1096
--- /dev/null
+++ b/postgres/schema/0017_natural-key-assignments.sql
@@ -0,0 +1,6 @@
+-- +goose Up
+ALTER TABLE assignments DROP id;
+ALTER TABLE assignments ADD PRIMARY KEY (category_id, date);
+
+-- +goose Down
+ALTER TABLE assignments ADD COLUMN id uuid DEFAULT uuid_generate_v4() PRIMARY KEY;
\ No newline at end of file