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

docs/cli.rst

@@ -619,6 +619,10 @@ If you omit the other table and other column references ``sqlite-utils`` will at
 
     $ sqlite-utils add-foreign-key books.db books author_id
 
+Add ``--ignore`` to ignore an existing foreign key (as opposed to returning an error)::
+
+    $ sqlite-utils add-foreign-key books.db books author_id --ignore
+
 See :ref:`python_api_add_foreign_key` in the Python API documentation for further details, including how the automatic table guessing mechanism works.
 
 .. _cli_add_foreign_keys:

docs/python-api.rst

@@ -858,6 +858,12 @@ The ``table.add_foreign_key(column, other_table, other_column)`` method takes th
 
 This method first checks that the specified foreign key references tables and columns that exist and does not clash with an existing foreign key. It will raise a ``sqlite_utils.db.AlterError`` exception if these checks fail.
 
+To ignore the case where the key already exists, use ``ignore=True``:
+
+.. code-block:: python
+
+    db["books"].add_foreign_key("author_id", "authors", "id", ignore=True)
+
 .. _python_api_add_foreign_keys:
 
 Adding multiple foreign key constraints at once

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(
+            if ignore:
-                "Foreign key already exists for {} => {}.{}".format(
+                return self
-                    column, other_table, other_column
+            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,

tests/test_cli.py

@@ -272,16 +272,24 @@ def test_add_foreign_key(db_path, args, assert_message):
             table="books", column="author_id", other_table="authors", other_column="id"
         )
     ] == db["books"].foreign_keys
+
     # Error if we try to add it twice:
     result = CliRunner().invoke(
         cli.cli, ["add-foreign-key", db_path, "books", "author_id", "authors", "id"]
     )
-
     assert 0 != result.exit_code
     assert (
         "Error: Foreign key already exists for author_id => authors.id"
         == result.output.strip()
     )
+
+    # No error if we add it twice with --ignore
+    result = CliRunner().invoke(
+        cli.cli,
+        ["add-foreign-key", db_path, "books", "author_id", "authors", "id", "--ignore"],
+    )
+    assert 0 == result.exit_code
+
     # Error if we try against an invalid column
     result = CliRunner().invoke(
         cli.cli, ["add-foreign-key", db_path, "books", "author_id", "authors", "bad"]

tests/test_create.py

@@ -5,6 +5,7 @@ from sqlite_utils.db import (
     AlterError,
     NoObviousTable,
     ForeignKey,
+    Table,
 )
 from sqlite_utils.utils import sqlite3
 import collections
@@ -348,7 +349,9 @@ def test_add_foreign_key(fresh_db):
         ]
     )
     assert [] == fresh_db["books"].foreign_keys
-    fresh_db["books"].add_foreign_key("author_id", "authors", "id")
+    t = fresh_db["books"].add_foreign_key("author_id", "authors", "id")
+    # Ensure it returned self:
+    assert isinstance(t, Table) and t.name == "books"
     assert [
         ForeignKey(
             table="books", column="author_id", other_table="authors", other_column="id"
@@ -379,6 +382,13 @@ def test_add_foreign_key_error_if_already_exists(fresh_db):
     assert "Foreign key already exists for author_id => authors.id" == ex.value.args[0]
 
 
+def test_add_foreign_key_no_error_if_exists_and_ignore_true(fresh_db):
+    fresh_db["books"].insert({"title": "Hedgehogs of the world", "author_id": 1})
+    fresh_db["authors"].insert({"id": 1, "name": "Sally"}, pk="id")
+    fresh_db["books"].add_foreign_key("author_id", "authors", "id")
+    fresh_db["books"].add_foreign_key("author_id", "authors", "id", ignore=True)
+
+
 def test_add_foreign_keys(fresh_db):
     fresh_db["authors"].insert_all(
         [{"id": 1, "name": "Sally"}, {"id": 2, "name": "Asheesh"}], pk="id"