What are activity entites in MS CRM

Activities are tasks that you or your teams perform when they contact customers, for example, sending letters or making telephone calls. You can create activities for yourselves, can assign them to someone else, or can share them with other users or teams. An activity is any action which can be entered on a calendar and has time dimensions (start time, stop time, due date, and duration) that help determine when the action occurred or is to occur. 

As per business requirements we can create our own custom activity entity. And, we can create 1:N relationship with any other entities as just like normal entity.

Below are the list of features available for  activity entity.

1. It has default offline availability.
2. Mail merge is disabled for activity entity.
3. Activity entity ownership can be either team-owned or user-owned. You cannot set it as organization level.
4. You cannot display activity entity in sales, marketing or any other areas.
5. By default Feedback, Notes,  Connections and Queues are enabled by default we cannot change them.
6. Knowledge management cannot be enabled.
7. By default primary field for activity entity is subject which is a single line of text with 200 max length. We cannot change it.
8. A custom activity entity must not have relationships with activities.

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Actions on entity records

Each entity supports several records like create, update, delete or assign a record to team or user,. But not all actions are allowed on all entity or attributes. We need to verify IsValidForCreate, IsValidateForUpdate or IsValidateForDelete fileds values to check to know whether we can perform create, update or delete operations on that entity or attributes.

Below are the list of actions we do perform on entity records.

Create	To create a record in an entity
Update	To update an attribute or attributes of a records
Delete	To delete a record
Retrieve	To retrieve a record of an entity
RetrieveMultiple	To retrieve collection of records
Assign	Changes ownership of a record
Share	Grant, modifies or revoke access to a record
Associate	To create a link between a record and collection of records between related entities
Disassociate	To remove a link between a record and collection of records between related entities
SetState	To set the state of a record

Relationships between entities

We can build three types of relationships between entities.

1. One-to-many
2. Many-to-One
3. Many-to-many

I would like explain these relationships with an example. Assume there are two entities called A and B. As per the requirement A record can contains more than one B records, that means we need to create a one-to-many relationship between A and B, once you create the relationship, one A entity related lookup field will be created automatically in B entity. So, for each record you are creating in B entity you will selecting the respective A entity related record. One each record of A entity, you can see all the associated records of entity B in associated view.

Many-to-one is same as One-to-many if we are following above process from B to A.

The records for a many-to-many relationships are stored in an intersect entity which contains the ID of both the referenced and referencing records. In Many to many relationships we need to establish a link between records by associate or disassociate.