--crossdb option for joining across databases (#1232)

* Test for cross-database join, refs #283
* Warn if --crossdb used with more than 10 DBs, refs #283
* latest.datasette.io demo of --crossdb joins, refs #283
* Show attached databases on /_memory page, refs #283
* Documentation for cross-database queries, refs #283

tests/test_crossdb.py

@@ -0,0 +1,75 @@
+from datasette.cli import cli
+from click.testing import CliRunner
+import urllib
+import sqlite3
+from .fixtures import app_client_two_attached_databases_crossdb_enabled
+
+
+def test_crossdb_join(app_client_two_attached_databases_crossdb_enabled):
+    app_client = app_client_two_attached_databases_crossdb_enabled
+    sql = """
+    select
+      'extra database' as db,
+      pk,
+      text1,
+      text2
+    from
+      [extra database].searchable
+    union all
+    select
+      'fixtures' as db,
+      pk,
+      text1,
+      text2
+    from
+      fixtures.searchable
+    """
+    response = app_client.get(
+        "/_memory.json?" + urllib.parse.urlencode({"sql": sql, "_shape": "array"})
+    )
+    assert response.status == 200
+    assert response.json == [
+        {"db": "extra database", "pk": 1, "text1": "barry cat", "text2": "terry dog"},
+        {"db": "extra database", "pk": 2, "text1": "terry dog", "text2": "sara weasel"},
+        {"db": "fixtures", "pk": 1, "text1": "barry cat", "text2": "terry dog"},
+        {"db": "fixtures", "pk": 2, "text1": "terry dog", "text2": "sara weasel"},
+    ]
+
+
+def test_crossdb_warning_if_too_many_databases(tmp_path_factory):
+    db_dir = tmp_path_factory.mktemp("dbs")
+    dbs = []
+    for i in range(11):
+        path = str(db_dir / "db_{}.db".format(i))
+        conn = sqlite3.connect(path)
+        conn.execute("vacuum")
+        dbs.append(path)
+    runner = CliRunner(mix_stderr=False)
+    result = runner.invoke(
+        cli,
+        [
+            "serve",
+            "--crossdb",
+            "--get",
+            "/",
+        ]
+        + dbs,
+        catch_exceptions=False,
+    )
+    assert (
+        "Warning: --crossdb only works with the first 10 attached databases"
+        in result.stderr
+    )
+
+
+def test_crossdb_attached_database_list_display(
+    app_client_two_attached_databases_crossdb_enabled,
+):
+    app_client = app_client_two_attached_databases_crossdb_enabled
+    response = app_client.get("/_memory")
+    for fragment in (
+        "databases are attached to this connection",
+        "<li><strong>fixtures</strong> - ",
+        "<li><strong>extra database</strong> - ",
+    ):
+        assert fragment in response.text