diff --git a/src/models/company_info.rs b/src/models/company_info.rs
index 2d6da7a..f45dd98 100644
--- a/src/models/company_info.rs
+++ b/src/models/company_info.rs
@@ -1,7 +1,9 @@
 use chrono::NaiveDateTime;
 use serde_derive::{Deserialize, Serialize};
+use diesel::prelude::*;
 
 use crate::schema::company_infos;
+use crate::DbConn;
 
 /// The company (information) model.
 ///
@@ -57,6 +59,21 @@ pub struct CompanyInfo {
     pub updated_at: NaiveDateTime,
 }
 
+impl CompanyInfo {
+    /// Returns the original/previous company information model (if any).
+    pub fn original(&self, conn: &DbConn) -> QueryResult<Option<CompanyInfo>> {
+        use diesel::associations::HasTable;
+
+        match self.original_id {
+            Some(original_id) => CompanyInfo::table()
+                .find(original_id)
+                .first(&**conn)
+                .optional(),
+            None => Ok(None),
+        }
+    }
+}
+
 /// The new company (information) model.
 ///
 /// This model represents new company information that can be inserted into the database.
diff --git a/src/models/customer.rs b/src/models/customer.rs
index eaa0fd2..ef57f8c 100644
--- a/src/models/customer.rs
+++ b/src/models/customer.rs
@@ -1,7 +1,10 @@
 use chrono::NaiveDateTime;
 use serde_derive::{Deserialize, Serialize};
+use diesel::prelude::*;
 
+use crate::models::{Invoice, Task};
 use crate::schema::customers;
+use crate::DbConn;
 
 /// The customer model.
 ///
@@ -38,6 +41,31 @@ pub struct Customer {
     pub updated_at: NaiveDateTime,
 }
 
+impl Customer {
+    /// Returns the invoices billed to the customer.
+    pub fn invoices(&self, conn: &DbConn) -> QueryResult<Vec<Invoice>> {
+        Invoice::belonging_to(self).load(&**conn)
+    }
+
+    /// Returns the project/tasks associated with the customer.
+    pub fn tasks(&self, conn: &DbConn) -> QueryResult<Vec<Task>> {
+        Task::belonging_to(self).load(&**conn)
+    }
+}
+
+#[object(Context = DbConn)]
+impl Customer {
+    /// Returns the invoices billed to the customer.
+    fn invoices(&self, context: &DbConn) -> FieldResult<Vec<Invoice>> {
+        self.invoices(context).map_err(Into::into)
+    }
+
+    /// Returns the project/tasks associated with the customer.
+    fn tasks(&self, context: &DbConn) -> FieldResult<Vec<Task>> {
+        self.tasks(context).map_err(Into::into)
+    }
+}
+
 /// The new customer model
 ///
 /// This model represents a new customer that can be inserted into the database.
diff --git a/src/models/invoice.rs b/src/models/invoice.rs
index 7915d75..1fd39da 100644
--- a/src/models/invoice.rs
+++ b/src/models/invoice.rs
@@ -1,8 +1,10 @@
 use chrono::NaiveDateTime;
 use serde_derive::{Deserialize, Serialize};
+use diesel::prelude::*;
 
-use crate::models::{CompanyInfo, Customer};
+use crate::models::{CompanyInfo, Customer, Task};
 use crate::schema::invoices;
+use crate::DbConn;
 
 /// The invoice model.
 ///
@@ -31,6 +33,29 @@ pub struct Invoice {
     pub updated_at: NaiveDateTime,
 }
 
+impl Invoice {
+    /// Returns the associated company info at the time of billing.
+    pub fn company_info(&self, conn: &DbConn) -> QueryResult<CompanyInfo> {
+        use diesel::associations::HasTable;
+
+        CompanyInfo::table()
+            .find(self.company_info_id)
+            .first(&**conn)
+    }
+
+    /// Returns the associated customer.
+    pub fn customer(&self, conn: &DbConn) -> QueryResult<Customer> {
+        use diesel::associations::HasTable;
+
+        Customer::table().find(self.customer_id).first(&**conn)
+    }
+
+    /// Returns the billed tasks included in the invoice.
+    pub fn tasks(&self, conn: &DbConn) -> QueryResult<Vec<Task>> {
+        Task::belonging_to(self).load(&**conn)
+    }
+}
+
 /// The new invoice model.
 ///
 /// This model represents an new invoice for a customer that can be inserted into the database.
diff --git a/src/models/task.rs b/src/models/task.rs
index 487c339..e55fcc9 100644
--- a/src/models/task.rs
+++ b/src/models/task.rs
@@ -1,8 +1,10 @@
 use chrono::NaiveDateTime;
 use serde_derive::{Deserialize, Serialize};
+use diesel::prelude::*;
 
-use crate::models::{Customer, Invoice};
+use crate::models::{Customer, Invoice, TimeEntry};
 use crate::schema::tasks;
+use crate::DbConn;
 
 /// The task (or project) model.
 ///
@@ -35,6 +37,30 @@ pub struct Task {
     pub updated_at: NaiveDateTime,
 }
 
+impl Task {
+    /// Returns the associated customer.
+    pub fn customer(&self, conn: &DbConn) -> QueryResult<Customer> {
+        use diesel::associations::HasTable;
+
+        Customer::table().find(self.customer_id).first(&**conn)
+    }
+
+    /// Returns the associated invoice (if billed)
+    pub fn invoice(&self, conn: &DbConn) -> QueryResult<Option<Invoice>> {
+        use diesel::associations::HasTable;
+
+        match self.invoice_id {
+            Some(invoice_id) => Invoice::table().find(invoice_id).first(&**conn).optional(),
+            None => Ok(None),
+        }
+    }
+
+    /// Returns the registered time entries.
+    pub fn time_entries(&self, conn: &DbConn) -> QueryResult<Vec<TimeEntry>> {
+        TimeEntry::belonging_to(self).load(&**conn)
+    }
+}
+
 /// The new task model.
 ///
 /// This model represents a new task (or project) of a customer that can be inserted into the
diff --git a/src/models/time_entry.rs b/src/models/time_entry.rs
index a00b8bd..1cbf767 100644
--- a/src/models/time_entry.rs
+++ b/src/models/time_entry.rs
@@ -1,8 +1,10 @@
 use chrono::NaiveDateTime;
 use serde_derive::{Deserialize, Serialize};
+use diesel::prelude::*;
 
 use crate::models::Task;
 use crate::schema::time_entries;
+use crate::DbConn;
 
 /// The time entry model.
 ///
@@ -29,6 +31,15 @@ pub struct TimeEntry {
     pub updated_at: NaiveDateTime,
 }
 
+impl TimeEntry {
+    /// Returns the task the entry is registered for.
+    pub fn customer(&self, conn: &DbConn) -> QueryResult<Task> {
+        use diesel::associations::HasTable;
+
+        Task::table().find(self.task_id).first(&**conn)
+    }
+}
+
 /// The new time entry model.
 ///
 /// This model represents a new registered amount of time that can be inserted into the database.