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

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"