[WIP]: 33 designing models

Models.md

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+# Entity-relationship diagram
+
+```mermaid
+erDiagram
+USER }o--o{ USER : contact
+USER ||--|| USER_SETTINGS : has
+GROUP ||--|| GROUP_SETTINGS : has
+USER ||--o{ MESSAGE : write
+MESSAGE }o--o| MESSAGE : "answer to"
+USER }o--o{ GROUP : "joined as role to"
+USER }o--o{ GROUP : "grouped groups into folder"
+MESSAGE }o--|| GROUP : "written in"
+MEDIA }o--|| MESSAGE : "attached to"
+```
+
+# Models
+
+`USER`: Represents individual users.  
+`GROUP`: Represents a group of users.  
+`USER_SETTINGS`: Represents user settings.
+`GROUP_SETTINGS`: Represents group settings.
+`MESSAGE`: Represents individual messages sent or received.  
+`MEDIA`: Represents various forms of media that can be attached to messages.
+
+# Relations
+
+## USER with USER
+
+Represents a many-to-many relationship between users, indicating that users can be in contact with multiple other users and vice versa. This could signify connections or relationships between users.
+
+## USER with USER_SETTINGS
+
+Represents a one-to-one relationship between USER and USER_SETTINGS, indicating that each user has one specific set of settings associated with them. Each user's settings are unique and directly linked to that user.
+
+## GROUP with GROUP_SETTINGS
+
+Represents a one-to-one relationship between GROUP and GROUP_SETTINGS, implying that each group has one specific set of settings associated with it.
+
+## USER with MESSAGE
+
+Represents a one-to-many relationship between USER and MESSAGE, indicating that a user can write multiple messages, but each message is written by only one user.
+
+## MESSAGE with MESSAGE
+
+Represents a many-to-one relationship between messages, indicating that many messages can be responses or answers to a single specific message.
+
+## USER with GROUP
+
+Represents a many-to-many relationship between users and groups, where users can be associated with multiple groups, and a group can have multiple users in various roles.
+
+## USER with GROUP
+
+Represents a many-to-many relationship between users and groups, indicating that users can organize or group multiple groups into various folders, and a folder can contain multiple groups.
+
+## MESSAGE with GROUP
+
+Represents a one-to-many relationship between MESSAGE and GROUP, indicating that a message can be associated with one or more groups.
+
+## MEDIA with GROUP
+
+Represents a many-to-one relationship between MEDIA and MESSAGE, indicating that a message can have multiple types of media attached to it, but each media attachment belongs to only one message.

README.md

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 # 🦾 Back-end part of the messenger application
+
+Detailed entities description can be found here [Models](Models.md)