use table_exists? to detect if migration table should be created to avoid SQLite3 error

[archiloque]

use table_exists? to detect if migration table should be created to avoid SQLite3::SQLException: index migrations_name_index already exists

[m-o-e]

Any reason this is not getting merged?

On postgres the error looks like this: PG::Error: ERROR: relation "migrations_name_index" already exists: CREATE INDEX "migrations_name_index" ON "migrations" ("name")