Diesel CLI is an optional tool Diesel provides to manage your database schema. Its main two roles are to run database migrations, and to create a Rust file which represents your database schema.

The behavior of Diesel CLI can be configured through a toml file. By default Diesel will look for diesel.toml in the same directory as your Cargo.toml file. You can provide a different config file by setting the DIESEL_CONFIG_FILE environment variable, or passing --config-file on the command line. You can get a basic config file with some defaults provided by running diesel setup.

As of Diesel 2.0, the file contains a single section, [print_schema]. All fields in this file are optional.

The file field

This field specifies the file where you want the Rust representation of your schema to live. When this field is present, commands which modify database schema (such as diesel migration run) will automatically run diesel print-schema, and output its results to this file.

This means that you can modify your database schema without having to worry about running a separate command to update your Rust code. It is highly recommended that you use this field, to ensure that the Rust representation of your database schema is always in sync with what is actually in your database. Typically this is set to src/schema.rs.

Unlike other fields, this doesn’t actually modify the behavior of diesel print-schema. diesel print-schema will always output your schema to stdout, regardless of whether this field is present or not.

The with_docs field

When this field is set to true, diesel print-schema will act as though the --with-docs flag was passed by default. This places a doc comment on all tables and columns.

The filter field

This field specifies which tables should be output by diesel print-schema. It corresponds to the --only-tables and --except-tables on the command line. Its value should be a map with one of those two keys. For example:

[print_schema]
# This will cause only the users and posts tables to be output
filter = { only_tables = ["users", "posts"] }

# This will cause all tables *except* the comments table to be
# output
filter = { except_tables = ["comments"] }

The schema field

Specifies which schema to use when searching for tables. When set, diesel print-schema will always behave as though --schema were passed. This field only affects PostgreSQL. If no value is provided, the public schema will be searched.

The generate_missing_sql_type_definitions field

This field accepts a boolean value, that configures whether diesel print-schema should generate definitions for missing sql types as part of the schema.rs file. The generated sql type definitions are placed in a separate sql_types module inside of the generated schema.rs file. This will generate only definitions for sql types actually used by any of the table! definitions generated by diesel print-schema. Those definitions automatically use the auto-generated sql type definition.

If this field is not present, or set to true diesel print-schema will generate the corresponding definitions. This configuration only generates custom sql type definition for PostgreSQL based database systems, as SQLite and MySQL are using a fixed set of sql types.

[print_schema]
# skip generating missing sql type definitions
generate_missing_sql_type_definitions = false

The custom_type_derives field

This field adds #[derive(...)] items to the sql type definitions automatically generated as a result of the generate_missing_sql_type_definitions field. When set, diesel print-schema will behave as if --custom-type-derives were passed. The diesel::sql_types::SqlType trait will be automatically derived if this flag is either empty, or doesn’t already contain the trait.

[print_schema]
generate_missing_sql_type_definitions = true
# Derive `SqlType` and `Debug` for the automatically generated sql type definitions
custom_type_derives = ["diesel::sql_types::SqlType", "std::fmt::Debug"]

The import_types field

This field adds use statements to the top of every table! declaration. When set, diesel print-schema will behave as if --import-types were passed. When no value is given, only types from diesel::sql_types will be imported.

[print_schema]
# Add types from `diesel_full_text_search` like `tsvector`
import_types = ["diesel::sql_types::*", "diesel_full_text_search::types::*"]

The patch_file field

Specifies a .patch file to be applied to your schema after it is generated. Corresponds to the --patch-file option on the command line.

We can’t provide an option for every possible customization to this file that you might want to make. This serves as a general purpose catch-all for schema customizations.

The file should be a unified diff, which you can generate with diff or git diff. It’s highly recommended that you provide more than 3 context lines, especially if you have set import_types.

You can easily generate this file by making the changes you want to schema.rs, and then running git diff -U6 > src/schema.patch.