Chore: Add Linter documentation

docs/concepts/linter.md

@@ -0,0 +1,243 @@
+# Linter guide
+
+Linting is a powerful tool for improving code quality and consistency. It enables you to automatically validate model definition, ensuring they adhere to your team's best practices.
+
+When a SQLMesh command is executed and the project is loaded, each model's code is checked for compliance with a set of rules you choose.
+
+SQLMesh provides built-in rules, and you can define custom rules. This improves code quality and helps detect issues early in the development cycle when they are simpler to debug.
+
+## Rules
+
+Each linting rule is responsible for identifying a pattern in a model's code.
+
+Some rules validate that a pattern is *not* present, such as not allowing `SELECT *` in a model's outermost query. Other rules validate that a pattern *is* present, like ensuring that every model's `owner` field is specified. We refer to both of these below as "validating a pattern".
+
+Rules are defined in Python. Each rule is an individual Python class that inherits from SQLMesh's `Rule` base class and defines the logic for validating a pattern.
+
+We display a portion of the `Rule` base class's code below ([full source code](https://github.com/TobikoData/sqlmesh/blob/main/sqlmesh/core/linter/rule.py)). Its methods and properties illustrate the most important components of the subclassed rules you define.
+
+Each rule class you create has four vital components:
+
+1. Name: the class's name is used as the rule's name.
+2. Description: the class should define a docstring that provides a short explanation of the rule's purpose.
+3. Pattern validation logic: the class should define a `check_model()` method containing the core logic that validates the rule's pattern. The method can access any `Model` attribute.
+4. Rule violation logic: if a rule's pattern is not validated, the rule is "violated" and the class should return a `RuleViolation` object. The `RuleViolation` object should include the contextual information a user needs to understand and fix the problem.
+
+``` python linenums="1"
+# Class name used as rule's name
+class Rule:
+    # Docstring provides rule's description
+    """The base class for a rule."""
+
+    # Pattern validation logic goes in `check_model()` method
+    @abc.abstractmethod
+    def check_model(self, model: Model) -> t.Optional[RuleViolation]:
+        """The evaluation function that checks for a violation of this rule."""
+
+    # Rule violation object returned by `violation()` method
+    def violation(self, violation_msg: t.Optional[str] = None) -> RuleViolation:
+        """Return a RuleViolation instance if this rule is violated"""
+        return RuleViolation(rule=self, violation_msg=violation_msg or self.summary)
+```
+
+### Built-in rules
+
+SQLMesh includes a set of predefined rules that check for potential SQL errors or enforce code style.
+
+An example of the latter is the `NoSelectStar` rule, which prohibits a model from using `SELECT *` in its query's outer-most select statement.
+
+Here is code for the built-in `NoSelectStar` rule class, with the different components annotated:
+
+``` python linenums="1"
+# Rule's name is the class name `NoSelectStar`
+class NoSelectStar(Rule):
+    # Docstring explaining rule
+    """Query should not contain SELECT * on its outer most projections, even if it can be expanded."""
+
+    def check_model(self, model: Model) -> t.Optional[RuleViolation]:
+        # If this model does not contain a SQL query, there is nothing to validate
+        if not isinstance(model, SqlModel):
+            return None
+
+        # Use the query's `is_star` property to detect the `SELECT *` pattern.
+        # If present, call the `violation()` method to return a `RuleViolation` object.
+        return self.violation() if model.query.is_star else None
+```
+
+Here are all of SQLMesh's built-in linting rules:
+
+| Name                       | Check type  | Explanation                                                                                                              |
+| -------------------------- | ----------- | ------------------------------------------------------------------------------------------------------------------------ |
+| ambiguousorinvalidcolumn  | Correctness | SQLMesh found duplicate columns or was unable to determine whether a column is duplicated or not                         |
+| invalidselectstarexpansion | Correctness | The query's top-level selection may be `SELECT *`, but only if SQLMesh can expand the `SELECT *` into individual columns |
+| noselectstar               | Stylistic   | The query's top-level selection may not be `SELECT *`, even if SQLMesh can expand the `SELECT *` into individual columns |
+
+
+### User-defined rules
+
+You may define custom rules to implement your team's best practices.
+
+For instance, you could ensure all models have an `owner` by defining the following linting rule:
+
+``` python linenums="1" title="linter/user.py"
+import typing as t
+
+from sqlmesh.core.linter.rule import Rule, RuleViolation
+from sqlmesh.core.model import Model
+
+class NoMissingOwner(Rule):
+    """Model owner should always be specified."""
+
+    def check_model(self, model: Model) -> t.Optional[RuleViolation]:
+        # Rule violated if the model's owner field (`model.owner`) is not specified
+        return self.violation() if not model.owner else None
+
+```
+
+Place a rule's code in the project's `linter/` directory. SQLMesh will load all subclasses of `Rule` from that directory.
+
+If the rule is specified in the project's [configuration file](#applying-linting-rules), SQLMesh will run it when the project is loaded. All SQLMesh commands will load the project, except for `create_external_models`, `migrate`, `rollback`, `run`, `environments`, and `invalidate`.
+
+SQLMesh will error if a model violates the rule, informing you which model(s) violated the rule. In this example, `full_model.sql` violated the `NoMissingOwner` rule:
+
+``` bash
+$ sqlmesh plan
+
+Linter errors for .../models/full_model.sql:
+ - nomissingowner: Model owner should always be specified.
+
+Error: Linter detected errors in the code. Please fix them before proceeding.
+```
+
+## Applying linting rules
+
+Specify which linting rules a project should apply in the project's [configuration file](../guides/configuration.md).
+
+Rules are specified as lists of rule names under the `linter` key. Globally enable or disable linting with the `enabled` key, which is `false` by default.
+
+NOTE: you **must** set the `enabled` key to `true` key to apply the project's linting rules.
+
+### Specific linting rules
+
+This example specifies that the `"ambiguousorinvalidcolumn"` and `"invalidselectstarexpansion"` linting rules should be enforced:
+
+=== "YAML"
+
+    ```yaml linenums="1"
+    linter:
+      enabled: true
+      rules: ["ambiguousorinvalidcolumn", "invalidselectstarexpansion"]
+    ```
+
+=== "Python"
+
+    ```python linenums="1"
+    from sqlmesh.core.config import Config, LinterConfig
+
+    config = Config(
+        linter=LinterConfig(
+            enabled=True,
+            rules=["ambiguousorinvalidcolumn", "invalidselectstarexpansion"]
+        )
+    )
+    ```
+
+### All linting rules
+
+Apply every built-in and user-defined rule by specifying `"ALL"` instead of a list of rules:
+
+=== "YAML"
+
+    ```yaml linenums="1"
+    linter:
+      enabled: True
+      rules: "ALL"
+    ```
+
+=== "Python"
+
+    ```python linenums="1"
+    from sqlmesh.core.config import Config, LinterConfig
+
+    config = Config(
+        linter=LinterConfig(
+            enabled=True,
+            rules="all",
+        )
+    )
+    ```
+
+If you want to apply all rules except for a few, you can specify `"ALL"` and list the rules to ignore in the `ignored_rules` key:
+
+=== "YAML"
+
+    ```yaml linenums="1"
+    linter:
+      enabled: True
+      rules: "ALL" # apply all built-in and user-defined rules and error if violated
+      ignored_rules: ["noselectstar"] # but don't run the `noselectstar` rule
+    ```
+
+=== "Python"
+
+    ```python linenums="1"
+    from sqlmesh.core.config import Config, LinterConfig
+
+    config = Config(
+        linter=LinterConfig(
+            enabled=True,
+            # apply all built-in and user-defined linting rules and error if violated
+            rules="all",
+             # but don't run the `noselectstar` rule
+            ignored_rules=["noselectstar"]
+        )
+    )
+    ```
+
+### Exclude a model from linting
+
+You can specify that a specific *model* ignore a linting rule by specifying `ignored_rules` in its `MODEL` block.
+
+This example specifies that the model `docs_example.full_model` should not run the `invalidselectstarexpansion` rule:
+
+```sql linenums="1"
+MODEL(
+  name docs_example.full_model,
+  ignored_rules: ["invalidselectstarexpansion"] # or "ALL" to turn off linting completely
+);
+```
+
+### Rule violation behavior
+
+Linting rule violations raise an error by default, preventing the project from running until the violation is addressed.
+
+You may specify that a rule's violation should not error and only log a warning by specifying it in the `warning_rules` key instead of the `rules` key.
+
+=== "YAML"
+
+    ```yaml linenums="1"
+    linter:
+      enabled: True
+      # error if `ambiguousorinvalidcolumn` rule violated
+      rules: ["ambiguousorinvalidcolumn"]
+      # but only warn if "invalidselectstarexpansion" is violated
+      warning_rules: ["invalidselectstarexpansion"]
+    ```
+
+=== "Python"
+
+    ```python linenums="1"
+    from sqlmesh.core.config import Config, LinterConfig
+
+    config = Config(
+        linter=LinterConfig(
+            enabled=True,
+            # error if `ambiguousorinvalidcolumn` rule violated
+            rules=["ambiguousorinvalidcolumn"],
+            # but only warn if "invalidselectstarexpansion" is violated
+            warning_rules=["invalidselectstarexpansion"],
+        )
+    )
+    ```
+
+SQLMesh will raise an error if the same rule is included in more than one of the `rules`, `warning_rules`, and `ignored_rules` keys since they should be mutually exclusive.

docs/concepts/models/overview.md

@@ -446,6 +446,12 @@ to `false` causes SQLMesh to disable query canonicalization & simplification. Th
 ### validate_query
 :   Whether the model's query will be validated at compile time. This attribute is `false` by default. Setting it to `true` causes SQLMesh to raise an error instead of emitting warnings. This will display invalid columns in your SQL statements along with models containing `SELECT *` that cannot be automatically expanded to list out all columns. This ensures SQL is verified locally before time and money are spent running the SQL in your data warehouse.
 
+!!! warning
+    This flag is deprecated as of v.0.159.7+ in favor of the [linter](../linter.md). To preserve validation during compilation, the [built-in rules](../linter.md#built-in) that check for correctness should be [configured](../../guides/configuration.md#linter) to error severity.
+
+### ignored_rules
+: Specifies which linter rules should be ignored/excluded for this model.
+
 ## Incremental Model Properties
 
 These properties can be specified in an incremental model's `kind` definition.

docs/guides/configuration.md

@@ -1108,6 +1108,11 @@ def grant_schema_usage(evaluator):
 
 As demonstrated in these examples, the `environment_naming_info` is available within the macro evaluator for macros invoked within the `before_all` and `after_all` statements. Additionally, the macro `this_env` provides access to the current environment name, which can be helpful for more advanced use cases that require fine-grained control over their behaviour.
 
+### Linting
+
+SQLMesh provides a linter that checks for potential issues in your models' code. Enable it and specify which linting rules to apply in the configuration file's `linter` key.
+
+Learn more about linting configuration on the [linting concepts page](../concepts/linter.md).
 
 ### Debug mode
 

docs/reference/model_configuration.md

@@ -39,7 +39,7 @@ Configuration options for SQLMesh model properties. Supported by all model kinds
 | `enabled`             | Whether the model is enabled. This attribute is `true` by default. Setting it to `false` causes SQLMesh to ignore this model when loading the project.                                                                                                                                                                                                      |       bool        |    N     |
 | `gateway`             | Specifies the gateway to use for the execution of this model. When not specified, the default gateway is used.                                                                                                                                                                                                       |       str        |    N     |
 | `optimize_query`             | Whether the model's query should be optimized. This attribute is `true` by default. Setting it to `false` causes SQLMesh to disable query canonicalization & simplification. This should be turned off only if the optimized query leads to errors such as surpassing text limit.                                                                                                                                                                                                      |       bool        |    N     |
-| `validate_query`             | Whether the model's query will be strictly validated at compile time. This attribute is `false` by default. Setting it to `true` causes SQLMesh to raise an error instead of emitting warnings. This will display invalid columns in your SQL statements along with models containing `SELECT *` that cannot be automatically expanded to list out all columns.                                                                                                                                                                                              |       bool        |    N     |
+| `ignored_rules`             |  A list of linter rule names (or "ALL") to be ignored/excluded for this model                                                                                                                                                                                             |       str \| array[str]        |    N     |
 
 ### Model defaults
 
@@ -123,7 +123,6 @@ The SQLMesh project-level `model_defaults` key supports the following options, d
 - on_destructive_change (described [below](#incremental-models))
 - audits (described [here](../concepts/audits.md#generic-audits))
 - optimize_query
-- validate_query
 - allow_partials
 - enabled
 - interval_unit

mkdocs.yml

@@ -32,6 +32,7 @@ nav:
     - SQLMesh tools:
       - guides/ui.md
       - guides/tablediff.md
+      - concepts/linter.md
       - guides/observer.md
   - Concepts:
     - concepts/overview.md