Tables Used.
   1. Employee Details table[Principle data]Or[Header Information]
EmployeeTable[Header].PNG
   2. Employee Contact details(Ph no on working days and on leave)[Line Item]
EmployeeContactDetails[Line]Sub.PNG
3.  Employee Photo Details.
EmployeePhoto[Sub].PNG
Structures Used
[But U can directly use Database tables to include structure to create Data types]
1. Employee Details.
EmployeeDetailsStructure.PNG

2.  Employee Address Info[Include structure in Emp Details table].
EmployeeDetailsAddressInclude.PNG

3.  Employee Contact details(Info like Contact no during working days and on leave).
EmployeeContactDetailsStructure.PNG

4. Employee Photo details.
EmployeePhotoStructure.PNG
Lets start rocket science 😆 ,
Here we go,
Create your simple gateway service, following provided steps.
using table and structure list below with Project Details.
     1. Log onto the SAP NetWeaver Gateway system.
     2. Open transaction SEGW.
     3. The SAP NetWeaver Gateway Service Builder opens.
     4. Create a new project.
    create.PNG
            5.  Enter project name and meaningful description.
               CreateProjectDetails.PNG
               Your project is created,

Entity Type
      6.  Right click on the Data Model folder and select  Import DDIC  Structure .
  1. DDIC.png
7. Following window will appear, In ABAP Structure field
          You can import the following DDIC structures
          into the Service Builder:
    • Views
    • Database tables
    • Structures
     8.  Press enter below window will appear,
     9.  Select at-least one key or more than one keys according to requirement
         in Usage column.
     10. You can directly create complex types by adding
         Deep structure (Structure within structure).
               /wp-content/uploads/2014/11/add_struc_577381.png
Entity Set
      11. Create Entity set for each Entity type.
Right click on Entity sets->select create
CreateEset.png

     12. Press F4 on Entity type field and select entity type,
                 Enter Entity Set name click okay,
          EntitySetCreate.PNG
     13.     Create Entity Sets for each entity type.
    
Association.
     14.     If you have Header Entity[Principal or Parent]
             and LineItem Entity[Dependent Information]
      Or
      If you want to create Relationship between 2 entities
          Ex: Purchase Order header and Line Items.
              Or
          Ex: Employee Details[Main] like id and name and Employee
              Contatct details[Dependent] many.
      15.      Right click on Association->create
          CreateAssoc.png
     16. Enter the Association name,
     17. Press F4 on Entity Type name and select Parent[Header Info]
         Entity type.
     18. Specify proper cardinality between entity types.
     19. Specify Navigation Property name[This will be used in URL to navigate from parent to child and child           to parent, In our case its only from parent to child] .
     AssociationWin.PNG
     20. Click next.
     21. Press F4 on Dependent property select the key field which is
         common among both.
     22. Click next.
  1. RelationShip.PNG
     23. Check Entries are proper.
     24. Click Finish,
          Next window.PNG
Once after creating Entity Types, Entity Sets, Associations, Navigation Properties.
Your Project View look like this.
     
Project Details

ProjectStructure.PNG
ProjectStucture2.PNG
     1. Click on Generate Runtime objects.
     GenerateRuntimeObject.PNG
     2. Expand Runtime artifacts.
     3. Right click on Class which ends with DPC_EXT and select Go to ABAP Workbench.
        Workbench.png
     4. Redefine the methods which mentioned below.
     5. Paste the code[Put break-point if u want to check behavior of methods called    based on URL].
     6. Expand Service maintenance.
     7. Right click on system in which you created these objects.
     8. Click on  Register and select proper System in RFC System alias.
     9. Right click again on Selected system and click maintain.
    10. Pop up will appear asking for redirection Click Yes.
    11. Click on Gateway Client.
Redirected.PNG
    12. Click Execute to test.
    13. Status 200 shows service is created correctly.
    14. Paste URL in Requested URI field and Click Execute. Check respective methods by keeping breakpoints               as given below.
GatewayClient.PNG
     15. Click Execute to test.
     Status 200 shows service is created correctly.
     16.Paste URL in Requested URI field and Click Execute.
     17.Check respective methods by keeping breakpoints as given below.
  
READ.
             
URL :
To get single record.
——————————/sap/ZEMPLOYEE_INFO_SRV/EmpDetailsSet(EMP_ID=’1′)
METHOD empdetailsset_get_entity.
DATAls_key_tab TYPE /iwbep/s_mgw_name_value_pair,
ls_emp_tab 
TYPE zxx_emp_det.
READ TABLE it_key_tab INTO ls_key_tab WITH KEY name ‘EMP_ID’.
SELECT SINGLE FROM zxx_emp_det INTO ls_emp_tab WHERE emp_id ls_key_tabvalue.
IF sysubrc 0.
MOVE-CORRESPONDING ls_emp_tab TO er_entity.
MOVE-CORRESPONDING ls_emp_tab TO er_entitydetails.
ENDIF.
ENDMETHOD.
SUCCESS CODE : 200
URL:
To get all records from table respect to EntitySet.
——————————-/sap/ZEMPLOYEE_INFO_SRV/EmpDetailsSet
To get records based on where condition.
——————————/sap/ZEMPLOYEE_INFO_SRV/EmpDetailsSet?$filter=EMP_ID eq ‘1’
We can add more conditions using & Operator in between fields.
METHOD empdetailsset_get_entityset.
DATAlt_emp_tab    TYPE TABLE OF zxx_emp_det,
ls_emp_tab    
TYPE          zxx_emp_det,
es_entityset  
TYPE          zcl_zemployee_info_mpc=>ts_empdetails.
SELECT FROM zxx_emp_det INTO TABLE lt_emp_tab WHERE (iv_filter_string).
IF sysubrc 0.
LOOP AT lt_emp_tab INTO ls_emp_tab.
MOVE-CORRESPONDING  ls_emp_tab    TO es_entityset.
MOVE-CORRESPONDING  ls_emp_tab    TO es_entitysetdetails.
APPEND              es_entityset  TO et_entityset.
CLEAR ls_emp_tab.
ENDLOOP.
ENDIF.
ENDMETHOD.
SUCCESS CODE : 200



CREATE

Steps :
  1. 1.Get any record from GET_ENTITY ..
  2. 2.After getting click on Use as request.
  3. 3.Change details what you want to create in left window.
  4. 4.Check POST method of HTTP.
  5. 5.Use URL..
—————————/sap/ZEMPLOYEE_INFO_SRV/EmpDetailsSet
method ZEMPDETSTRSET_CREATE_ENTITY.
DATAls_emp_details TYPE zxx_emp_det.
“Get the created entity here
io_data_provider
->read_entry_dataIMPORTING es_data er_entity ).
MOVE-CORRESPONDING er_entity TO ls_emp_details.
MOVE-CORRESPONDING er_entitydetails TO ls_emp_details.
INSERT zxx_emp_det FROM ls_emp_detailsendmethod.
SUCCESS CODE : 201


UPDATE
To Update the Record.
Follow same steps as for CREATE_ENTITY.
Just Change URL and Check HTTP  PUT Method.
  1. URL.
—————————/sap/ZEMPLOYEE_INFO_SRV/EmpDetailsSet(EMP_ID=’1′)
But on response   body nothing will be displayed..
For that we need to call get entity internally.
method EMPDETAILSSET_UPDATE_ENTITY.
DATAls_emp_details TYPE zxx_emp_det.
“Get the created entity here
io_data_provider
->read_entry_dataIMPORTING es_data er_entity ).
MOVE-CORRESPONDING er_entity TO ls_emp_details.
MOVE-CORRESPONDING er_entitydetails TO ls_emp_details.
MODIFY zxx_emp_det FROM ls_emp_details.
endmethod.
SUCCESS CODE : 204


DELETE

To delete record Follow same steps for create.
Check http method DELETE.
URL .
——————–/sap/ZEMPLOYEE_INFO_SRV/EmpDetailsSet(EMP_ID=’1′)
method EMPDETAILSSET_DELETE_ENTITY.
DATAls_key_tab TYPE /iwbep/s_mgw_name_value_pair,
ls_emp_details 
TYPE zxx_emp_det.
READ TABLE it_key_tab INTO ls_key_tab WITH KEY name ‘EMP_ID’.
IF sysubrc 0.
ls_emp_details
emp_id   ls_key_tabvalue.
DELETE zxx_emp_det FROM ls_emp_details.
ENDIF.
endmethod.
SUCCESS CODE : 204
  
DEEP READ

To get data of dependent with parent entity[Header with Line Items]
  1. URL.
—————————–ZEMPLOYEE_INFO_SRV/EmpDetailsSet(EMP_ID=’1′)/?$expand=EmpContDetailsNavig
METHOD /iwbep/if_mgw_appl_srv_runtime~get_expanded_entity.
DATA:   ls_key_tab        TYPE /iwbep/s_mgw_name_value_pair,
ls_emp_tab        
TYPE zxx_emp_det,
ls_empadd_tab     
TYPE zot_cont_det,
ls_empadd         
TYPE zot_cont_det.
DATAls_expand_tech_clauses LIKE LINE OF et_expanded_tech_clauses.
DATABEGIN OF ls_expand.
INCLUDE TYPE zcl_zemployee_info_mpc=>ts_empdetails.
DATAempcontdetailsnavig TYPE STANDARD TABLE OFzcl_zemployee_info_mpc=>ts_empcontactdetails WITH DEFAULT KEY.
DATAEND OF ls_expand.
READ TABLE it_key_tab INTO ls_key_tab WITH KEY name ‘EMP_ID’.
SELECT SINGLE FROM zxx_emp_det INTO ls_emp_tab WHERE emp_id ls_key_tabvalue.
SELECT SINGLE FROM zot_cont_det INTO ls_empadd_tab WHERE emp_id ls_key_tabvalue.
MOVE-CORRESPONDING ls_emp_tab TO ls_expand.
MOVE-CORRESPONDING ls_empadd_tab TO ls_empadd.
APPEND ls_empadd TO ls_expandempcontdetailsnavig.
“Assign the navigation properties
ls_expand_tech_clauses 
’empcontdetailsnavig’.
APPEND ls_expand_tech_clauses TO et_expanded_tech_clauses.
copy_data_to_ref(
EXPORTING
is_data 
ls_expandCHANGING
cr_data 
er_entity ).
ENDMETHOD.
SUCCESS CODE : 200



DEEP INSERT

To create deep entity.[Parent data with child][Header with Line Items]
Steps :
  1. 1.Read data from GET_EXPANDED_ENTITY
  2. 2.Made HTTP method as POST.
  3. 3.Click User as Request and Edit values You want to Insert.
  4. 4.url
———————–ZEMPLOYEE_INFO_SRV/EmpDetailsSet
  1. 5.Execute.
method /IWBEP/IF_MGW_APPL_SRV_RUNTIME~CREATE_DEEP_ENTITY.
TYPES ty_t_empadd TYPE STANDARD TABLE OFZCL_ZEMPLOYEE_INFO_MPC=>TS_EMPCONTACTDETAILS WITH DEFAULT KEY.
TYPESBEGIN OF ty_expand.
INCLUDE TYPE ZCL_ZEMPLOYEE_INFO_MPC=>TS_EMPDETAILS.
TYPES  :       EmpContDetailsNavig TYPE ty_t_empadd,
END OF ty_expand.
DATA :
*&——-> Local structures.
ls_deep_emp_details 
TYPE  ty_expand,
ls_emp_cont_det     
TYPE  ZOT_CONT_DET,
ls_emp_details      
TYPE  zxx_emp_det,
*&——-> Local Itabs.
lt_emp_cont_det     
TYPE TABLE OF ZOT_CONT_DET.
io_data_provider->read_entry_dataIMPORTING es_data ls_deep_emp_details ).
copy_data_to_refEXPORTING  is_data ls_deep_emp_detailsCHANGING   cr_data er_deep_entity ).
MOVE-CORRESPONDING ls_deep_emp_details TO ls_emp_details.
MOVE-CORRESPONDING ls_deep_emp_detailsdetails TO ls_emp_details.
INSERT INTO zxx_emp_det VALUES ls_emp_details.
LOOP AT ls_deep_emp_detailsempcontdetailsnavig INTO ls_emp_cont_det.
APPEND ls_emp_cont_det TO lt_emp_cont_det.
ENDLOOP.
MODIFY zot_cont_det FROM TABLE lt_emp_cont_det.
IF sysubrc EQ 4.
ENDIF.
endmethod.
SUCCESS CODE : 201



CREATE MEDIA

  1. 1.Redefine the DEFINE METHOD OF MPC_EXT CLASS
  2. 2.Paste code.
  3. 3.We need to get key values in SLUG Parameter Of http.
  4. URL.
——————————-ZEMP_PRO_SRV/EmpDetailsSet(‘1’)/EmpPhotoNavig
  1. 4.Click on add file select image.
  2. 5.Check Post method of HTTP.
  3. 5.Execute.
method DEFINE.
super
->DEFINE( ).
DATA:
lo_entity   
type REF TO /IWBEP/IF_MGW_ODATA_ENTITY_TYP,
lo_property 
type REF TO /IWBEP/IF_MGW_ODATA_PROPERTY.
lo_entity 
model->GET_ENTITY_TYPEIV_ENTITY_NAME ‘EmpPhoto’ ).
IF lo_entity is BOUND.
lo_property 
lo_entity->GET_PROPERTYIV_PROPERTY_NAME ‘MIMETYPE’ ).
lo_property
->SET_AS_CONTENT_TYPE( ).
ENDIF.
endmethod.
method /IWBEP/IF_MGW_APPL_SRV_RUNTIME~CREATE_STREAM.
   DATAls_key_tab                   TYPE /iwbep/s_mgw_name_value_pair,
         lt_key_tab                   TYPE /iwbep/t_mgw_name_value_pair,
         ls_photo                     TYPE zemp_photos,
         lv_email                     TYPE char100.
   CASE iv_entity_name.
     WHEN ‘EmpPhoto’.
       READ TABLE it_key_tab WITH KEY name ‘EMP_ID’ INTO ls_key_tab.
       IF SYSUBRC EQ 0.
         ls_photoemp_id ls_key_tabvalue.
       ENDIF.
       ls_photomimetype is_media_resourcemime_type.
       ls_photofilename iv_slug.
       ls_photocontent is_media_resourcevalue.
       DELETE FROM zemp_photos WHERE emp_id ls_photoemp_id.
       INSERT INTO zemp_photos VALUES ls_photo.
       empphotoset_get_entity(
         EXPORTING
           iv_entity_name     iv_entity_name
           iv_entity_set_name iv_entity_set_name
           iv_source_name     iv_source_name
           it_key_tab         it_key_tab
           it_navigation_path it_navigation_path
         IMPORTING
           er_entity          ls_photo ).
       copy_data_to_refEXPORTING is_data ls_photo
                         CHANGING  cr_data er_entity ).
   ENDCASE.
endmethod.
  
READ MEDIA

Steps:
URL:
———————/ZEMPLOYEE_INFO_SRV/EmpPhotoSet(‘1’)/$value
  1. 1.Redefine GET_STREAM Method Paste code.
  2. 2.Select GET Method of HTTP.
  3. 3.Execute.
method /IWBEP/IF_MGW_APPL_SRV_RUNTIME~GET_STREAM.
    DATAls_key      TYPE /iwbep/s_mgw_name_value_pair,
lv_email    
TYPE char100,
ls_photo    
TYPE ZEMP_PHOTOS,
ls_lheader  
TYPE ihttpnvp,
ls_stream   
TYPE ty_s_media_resource,
lv_filename 
TYPE w3conttype.
CASE iv_entity_name.
WHEN ‘EmpPhoto’.
READ TABLE it_key_tab WITH KEY name ‘EMP_ID’ INTO ls_key.
lv_email 
ls_keyvalue.
SELECT SINGLE FROM ZEMP_PHOTOS INTO CORRESPONDING FIELDS OF ls_photo WHEREEMP_ID lv_email.
ls_stream
value ls_photocontent.
ls_stream
mime_type ls_photomimetype.
lv_filename ls_photofilename.
*        lv_filename = escape( val = lv_filename
*                              format = cl_abap_format=>e_url ).
*        ls_lheader-name = ‘Content-Disposition’.
*        ls_lheader-value = |inline; filename=”{ lv_filename }”|.
*        set_header( is_header = ls_lheader ).
ls_streamvalue ls_photocontent.
ls_stream
mime_type lv_filename.
copy_data_to_refEXPORTING is_data ls_streamCHANGING  cr_data er_stream ).
*        copy_data_to_ref( EXPORTING is_data = ls_stream
*                          CHANGING  cr_data = er_stream ).
ENDCASE.
  
***** SOME DUMMY REFERENCE CODE FOR SAME METHOD [CAN BE USEFUL]  ****

**TRY.
*CALL METHOD SUPER->/IWBEP/IF_MGW_APPL_SRV_RUNTIME~GET_STREAM
*  EXPORTING
*    IV_ENTITY_NAME          =
*    IV_ENTITY_SET_NAME      =
*    IV_SOURCE_NAME          =
*    IT_KEY_TAB              =
*    IT_NAVIGATION_PATH      =
**    io_tech_request_context =
**  IMPORTING
**    er_stream               =
*    .
** CATCH /iwbep/cx_mgw_busi_exception .
** CATCH /iwbep/cx_mgw_tech_exception .
**ENDTRY.
*DATA: ls_stream    TYPE         ty_s_media_resource
*      ,lo_api    TYPE REF TO  if_mr_api
*      ,lv_entity_name TYPE  /IWBEP/MGW_TECH_NAME
*      ,er_entity type REF TO DATA
*      ,lr_entity TYPE REF TO DATA
*      ,empphotoset_get_entity TYPE ZCL_ZEMP_PRO_MPC=>TS_EMPPHOTO
*      ,ls_content type table of TBL1024 initial size 0
*      ,ls_xstring type xstring
*      ,lt_messg type table of BAPIRET2 initial SIZE 0
*      ,lv_pernr(8) TYPE n
*      ,t_photo type table of TBL1024
*      ,l_photo type XSTRING
*      ,l_line type string
*      ,l_photo1 type string
*      ,ls_ret type BAPIRET2
*      ,ls_photo type ZEMP_PHOTOS
*      ,t_msg type table of BAPIRET2.
*
*lv_entity_name = io_tech_request_context->GET_ENTITY_TYPE_NAME( ).
*
*CASE lv_entity_name.
*
*      WHEN ‘ZsempDet’.
*      WHEN ‘EmpPhoto’.
*
*     empphotoset_get_entity(
*           EXPORTING iv_entity_name     = iv_entity_name
*                     iv_entity_set_name = iv_entity_set_name
*                     iv_source_name     = iv_source_name
*                     it_key_tab         = it_key_tab
*                     it_navigation_path = it_navigation_path
*                     io_tech_request_context = io_tech_request_context
*           IMPORTING er_entity          = empphotoset_get_entity
*                                 ).
*
*           IF empphotoset_get_entity IS NOT INITIAL.
**     Send specific entity data to the caller interface
*           copy_data_to_ref(
*                     EXPORTING
*                     is_data = empphotoset_get_entity
*                     CHANGING
*                     cr_data = er_entity
*                          ).
*           ELSE.
**         In case of initial values – unbind the entity reference
*           er_entity = lr_entity.
*           ENDIF.
*
*           lv_pernr = empphotoset_get_entity-emp_id.
*
*           CALL FUNCTION ‘PAD_PHOTO_UPDATE_GET_DETAIL’
*           EXPORTING
*                IV_EMPLOYEE_NUMBER         = lv_pernr
*
*           TABLES
*                T_PHOTO_ARCHIVE_OUT        = t_photo
*                T_MESSAGES_OUT             = t_msg
*                .
*
**           LOOP AT t_photo into  ls_photo.
**                l_line = ls_photo-line.
**                concatenate l_photo1 l_line into l_photo1.
**           ENDLOOP.
*
*               ls_stream-value = l_photo = l_photo1.
*               ls_stream-mime_type = ‘image/jpeg’.
*
*               copy_data_to_ref( EXPORTING is_data = ls_stream
*                                 CHANGING  cr_data = er_stream ).
*
*      WHEN OTHERS.
*ENDCASE.
endmethod.
Entity Types.
Entity Sets.
  1. Associations.
Navigation Properties.