Improve documentation for the drivers and the entity

Provide module level code example on how to create an Entity, Model, ActiveModel, Column and PrimaryKey

src/entity/active_model.rs

@@ -55,7 +55,27 @@ where
     ActiveValue::unchanged(value)
 }
 
-/// Enforces a set of constraints on any type performing an Create, Update or Delete operation
+/// Enforces a set of constraints on any type performing an Create, Update or Delete operation.
+/// The type must also implement the [EntityTrait].
+/// #### Example
+/// ```
+/// use sea_orm::entity::prelude::*;
+///
+/// #[derive(Copy, Clone, Default, Debug, DeriveEntity)]
+/// pub struct Entity;
+///
+/// impl EntityName for Entity {
+///     fn table_name(&self) -> &str {
+///         "cake"
+///     }
+/// }
+///
+/// #[derive(Clone, Debug, PartialEq, DeriveActiveModel)]
+/// pub struct Model {
+///     pub id: i32,
+///     pub name: Option<String> ,
+/// }
+/// ```
 #[async_trait]
 pub trait ActiveModelTrait: Clone + Debug {
     /// Enforce the type to the constraints of the [EntityTrait]
@@ -186,7 +206,21 @@ pub trait ActiveModelTrait: Clone + Debug {
 }
 
 /// Enforce a set of constraints to a override the ActiveModel behavior
-/// Behaviors for users to override
+/// Behaviors for users to override.
+/// The type must also implement the [ActiveModelTrait]
+///
+/// ### Example
+/// ```
+/// /// Derive the active
+/// #[derive(Clone, Debug, PartialEq, DeriveModel, DeriveActiveModel)]
+/// pub struct Model {
+///     pub id: i32,
+///     pub name: String,
+///     pub cake_id: Option<i32>,
+/// }
+/// ```
+/// impl ActiveModelBehavior for ActiveModel {}
+///
 #[allow(unused_variables)]
 pub trait ActiveModelBehavior: ActiveModelTrait {
     /// Create a new ActiveModel with default values. Also used by `Default::default()`.

src/entity/base_entity.rs

@@ -37,6 +37,7 @@ pub trait EntityName: IdenStatic + Default {
     }
 }
 
+/// FIXME Add docs for manual impl
 /// An Entity implementing `EntityTrait` represents a table in a database.
 ///
 /// This trait provides an API for you to inspect it's properties

src/entity/mod.rs

@@ -1,3 +1,125 @@
+//! This modules contains types and traits for an  Entity, ActiveMode, Model, PrimaryKey, ForeignKey and Relations.
+//!
+//! // An Entity
+//! A unit struct implements [EntityTrait](crate::EntityTrait) representing a table in the database.
+//!
+//! This trait contains the properties of an entity including
+//!
+//!     Table Name (implemented EntityName)
+//!     Column (implemented ColumnTrait)
+//!     Relation (implemented RelationTrait)
+//!     Primary Key (implemented PrimaryKeyTrait and PrimaryKeyToColumn)
+//!
+//! This trait also provides an API for CRUD actions
+//!
+//!     Select: find, find_*
+//!     Insert: insert, insert_*
+//!     Update: update, update_*
+//!     Delete: delete, delete_*
+//!
+//! #### Example for creating an Entity, Model and ActiveModel
+//! ```
+//! use sea_orm::entity::prelude::*;
+//!
+//! // Use [DeriveEntity] to derive the EntityTrait automatically
+//! #[derive(Copy, Clone, Default, Debug, DeriveEntity)]
+//! pub struct Entity;
+//!
+//! /// The [EntityName] describes the name of a table
+//! impl EntityName for Entity {
+//!     fn table_name(&self) -> &str {
+//!         "cake"
+//!     }
+//! }
+//!
+//! // Create a Model for the Entity through [DeriveModel].
+//! // The `Model` handles `READ` operations on a table in a database.
+//! // The [DeriveActiveModel] creates a way to perform `CREATE` , `READ` and `UPDATE` operations
+//! // in a database
+//!
+//! #[derive(Clone, Debug, PartialEq, DeriveModel, DeriveActiveModel)]
+//! pub struct Model {
+//!     pub id: i32,
+//!     pub name: Option<String> ,
+//! }
+//!
+//! // Use the [DeriveColumn] to create a Column for an the table called Entity
+//! // The [EnumIter] which creates a new type that iterates of the variants of a Column.
+//! #[derive(Copy, Clone, Debug, EnumIter, DeriveColumn)]
+//! pub enum Column {
+//!     Id,
+//!     Name,
+//! }
+//!
+//! // Create a PrimaryKey for the Entity using the [PrimaryKeyTrait]
+//! // The [EnumIter] which creates a new type that iterates of the variants of a PrimaryKey.
+//! #[derive(Copy, Clone, Debug, EnumIter, DerivePrimaryKey)]
+//! pub enum PrimaryKey {
+//!     Id,
+//! }
+//!
+//! // Or implement the [PrimaryKeyTrait] manually instead of using the macro [DerivePrimaryKey]
+//! impl PrimaryKeyTrait for PrimaryKey {
+//!     type ValueType = i32;
+//!
+//!     fn auto_increment() -> bool {
+//!         true
+//!     }
+//! }
+//!
+//! // Create a Relation for the Entity
+//! #[derive(Copy, Clone, Debug, EnumIter)]
+//! #[sea_orm(
+//!        // The relation belongs to `Entity` type
+//!        belongs_to = "Entity",
+//!        from = "Column::FruitId",
+//!        to = "Column::Id"
+//!    )]
+//! pub enum Relation {
+//!     Fruit,
+//! }
+//!
+//! // Create the properties of a Column in an Entity ensuring that calling
+//! // Column::def() yields a Column definition as defined in [ColumnDef]
+//! impl ColumnTrait for Column {
+//!     type EntityName = Entity;
+//!     fn def(&self) -> ColumnDef {
+//!         match self {
+//!             Self::Id => ColumnType::Integer.def(),
+//!             Self::Name => ColumnType::Text.def().null(),
+//!         }
+//!     }
+//! }
+//!
+//! // Implement the set of constraints for creating a Relation as defined in the [RelationTrait]
+//! impl RelationTrait for Relation {
+//!     fn def(&self) -> RelationDef {
+//!         match self {
+//!             Self::Fruit => Entity::has_many(super::fruit::Entity).into(),
+//!         }
+//!     }
+//! }
+//!
+//! impl Related<fruit::Entity> for Entity {
+//!     fn to() -> RelationDef {
+//!         Relation::Fruit.def()
+//!     }
+//! }
+//!
+//! impl Related<super::filling::Entity> for Entity {
+//!     fn to() -> RelationDef {
+//!         super::cake_filling::Relation::Filling.def()
+//!     }
+//!     fn via() -> Option<RelationDef> {
+//!         Some(super::cake_filling::Relation::Cake.def().rev())
+//!     }
+//! }
+//!
+//! // Implement user defined operations for CREATE, UPDATE and DELETE operations
+//! // to create an ActiveModel using the [ActiveModelBehavior]
+//! impl ActiveModelBehavior for ActiveModel {}
+//! ```
+
 mod active_model;
 mod base_entity;
 mod column;

src/entity/primary_key.rs

@@ -4,7 +4,38 @@ use sea_query::{FromValueTuple, IntoValueTuple};
 use std::fmt::Debug;
 
 //LINT: composite primary key cannot auto increment
-/// A set of constraints to be used to define a Primary Key
+/// A set of constraints to be used to define a Primary Key.
+///
+/// A primary key can be derived manually
+///
+/// ### Example
+/// ```
+/// use sea_orm::entity::prelude::*;
+///
+/// #[derive(Copy, Clone, Debug, EnumIter)]
+/// pub enum PrimaryKey {
+///     Id,
+/// }
+/// impl PrimaryKeyTrait for PrimaryKey {
+///     type ValueType = i32;
+///
+///     fn auto_increment() -> bool {
+///         true
+///     }
+/// }
+/// ```
+///
+/// Alternatively, use derive macros to automatically implement the trait for a Primary Key
+///
+/// ### Example
+/// ```
+/// use sea_orm::entity::prelude::*;
+///
+/// #[derive(Copy, Clone, Debug, EnumIter, DerivePrimaryKey)]
+/// pub enum PrimaryKey {
+///     Id,
+/// }
+/// ```
 pub trait PrimaryKeyTrait: IdenStatic + Iterable {
     #[allow(missing_docs)]
     type ValueType: Sized