ignore=True argument for add_foreign_key, closes #112

Also --ignore for add-foreign-key command

Plus table.add_foreign_key(...) now returns self, allowing more chaining

sqlite_utils/cli.py

@@ -308,7 +308,12 @@ def add_column(path, table, col_name, col_type, fk, fk_col, not_null_default):
 @click.argument("column")
 @click.argument("other_table", required=False)
 @click.argument("other_column", required=False)
-def add_foreign_key(path, table, column, other_table, other_column):
+@click.option(
+    "--ignore",
+    is_flag=True,
+    help="If foreign key already exists, do nothing",
+)
+def add_foreign_key(path, table, column, other_table, other_column, ignore):
     """
     Add a new foreign key constraint to an existing table. Example usage:
 
@@ -318,7 +323,7 @@ def add_foreign_key(path, table, column, other_table, other_column):
     """
     db = sqlite_utils.Database(path)
     try:
-        db[table].add_foreign_key(column, other_table, other_column)
+        db[table].add_foreign_key(column, other_table, other_column, ignore=ignore)
     except AlterError as e:
         raise click.ClickException(e)
 

sqlite_utils/db.py

@@ -781,7 +781,9 @@ class Table(Queryable):
         else:
             return pks[0].name
 
-    def add_foreign_key(self, column, other_table=None, other_column=None):
+    def add_foreign_key(
+        self, column, other_table=None, other_column=None, ignore=False
+    ):
         # Ensure column exists
         if column not in self.columns_dict:
             raise AlterError("No such column: {}".format(column))
@@ -806,12 +808,16 @@ class Table(Queryable):
             and fk.other_table == other_table
             and fk.other_column == other_column
         ):
-            raise AlterError(
-                "Foreign key already exists for {} => {}.{}".format(
-                    column, other_table, other_column
+            if ignore:
+                return self
+            else:
+                raise AlterError(
+                    "Foreign key already exists for {} => {}.{}".format(
+                        column, other_table, other_column
+                    )
                 )
-            )
         self.db.add_foreign_keys([(self.name, column, other_table, other_column)])
+        return self
 
     def enable_fts(
         self,